--[[--
This module detects gestures.

Current detectable gestures:

* `touch` (user touched screen)
* `tap` (touch action detected as single tap)
* `pan`
* `hold`
* `swipe`
* `pinch`
* `spread`
* `rotate`
* `hold_pan`
* `double_tap`
* `inward_pan`
* `outward_pan`
* `pan_release`
* `hold_release`
* `two_finger_tap`
* `two_finger_pan`
* `two_finger_swipe`
* `two_finger_pan_release`

You change the state machine by feeding it touch events, i.e. calling
@{GestureDetector:feedEvent|GestureDetector:feedEvent(tev)}.


a touch event should have following format:

    tev = {
        slot = 1,
        id = 46,
        x = 0,
        y = 1,
        timev = time.s(123.23),
    }

Don't confuse `tev` with raw evs from kernel, `tev` is built according to ev.

@{GestureDetector:feedEvent|GestureDetector:feedEvent(tev)} will return a
detection result when you feed a touch release event to it.
--]]

local Geom = require("ui/geometry")
local logger = require("logger")
local time = require("ui/time")
local util = require("util")

-- We're going to need some clockid_t constants
local ffi = require("ffi")
local C = ffi.C
require("ffi/posix_h")

-- default values (time parameters are in milliseconds (ms))
local TAP_INTERVAL_MS = 0
local DOUBLE_TAP_INTERVAL_MS = 300
local TWO_FINGER_TAP_DURATION_MS = 300
local HOLD_INTERVAL_MS = 500
local SWIPE_INTERVAL_MS = 900

local GestureDetector = {
    -- must be initialized with the Input singleton class
    input = nil,
    -- default values (accessed for display by plugins/gestures.koplugin)
    TAP_INTERVAL_MS = TAP_INTERVAL_MS,
    DOUBLE_TAP_INTERVAL_MS = DOUBLE_TAP_INTERVAL_MS,
    TWO_FINGER_TAP_DURATION_MS = TWO_FINGER_TAP_DURATION_MS,
    HOLD_INTERVAL_MS = HOLD_INTERVAL_MS,
    SWIPE_INTERVAL_MS = SWIPE_INTERVAL_MS,
    -- pinch/spread direction table
    DIRECTION_TABLE = {
        east = "horizontal",
        west = "horizontal",
        north = "vertical",
        south = "vertical",
        northeast = "diagonal",
        northwest = "diagonal",
        southeast = "diagonal",
        southwest = "diagonal",
    },
    -- states are stored in separated slots
    states = {},
    pending_hold_timer = {},
    track_ids = {},
    tev_stacks = {},
    -- latest touch events fed in each slot
    last_tevs = {},
    first_tevs = {},
    -- for multiswipe gestures
    multiswipe_directions = {},
    -- detecting status on each slot
    detectings = {},
    -- for single/double tap
    last_taps = {},
    -- for timestamp clocksource detection
    clock_id = nil,
    -- current values
    ges_tap_interval = time.ms(G_reader_settings:readSetting("ges_tap_interval_ms") or TAP_INTERVAL_MS),
    ges_double_tap_interval = time.ms(G_reader_settings:readSetting("ges_double_tap_interval_ms")
        or DOUBLE_TAP_INTERVAL_MS),
    ges_two_finger_tap_duration = time.ms(G_reader_settings:readSetting("ges_two_finger_tap_duration_ms")
        or TWO_FINGER_TAP_DURATION_MS),
    ges_hold_interval = time.ms(G_reader_settings:readSetting("ges_hold_interval_ms") or HOLD_INTERVAL_MS),
    ges_swipe_interval = time.ms(G_reader_settings:readSetting("ges_swipe_interval_ms") or SWIPE_INTERVAL_MS),
}

function GestureDetector:new(o)
    o = o or {}
    setmetatable(o, self)
    self.__index = self
    if o.init then o:init() end
    return o
end

function GestureDetector:init()
    local scaler = self.screen:getDPI() / 167
    -- distance parameters
    self.TWO_FINGER_TAP_REGION = 20 * scaler
    self.DOUBLE_TAP_DISTANCE = 50 * scaler
    self.SINGLE_TAP_BOUNCE_DISTANCE = self.DOUBLE_TAP_DISTANCE
    self.PAN_THRESHOLD = self.DOUBLE_TAP_DISTANCE
    self.MULTISWIPE_THRESHOLD = self.DOUBLE_TAP_DISTANCE
end

--[[--
Feeds touch events to state machine.

Note that, in a single input frame, if the same slot gets multiple events, only the last one is kept.
Every slot in the input frame is consumed, and that in FIFO order (slot order based on appearance in the frame).
--]]
function GestureDetector:feedEvent(tevs)
    local gestures = {}
    for _, tev in ipairs(tevs) do
        local slot = tev.slot
        if not self.states[slot] then
            self:clearState(slot) -- initialize slot state
        end
        local ges = self.states[slot](self, tev)
        if tev.id ~= -1 then
            -- NOTE: tev is actually a simple reference to Input's self.ev_slots[slot],
            --       which means self.last_tevs[slot] doesn't actually point to the *previous*
            --       input frame for a given slot, but always points to the *current* input frame for that slot!
            --       Compare to self.first_tevs below, which does create a copy...
            self.last_tevs[slot] = tev
        end
        if ges then
            table.insert(gestures, ges)
        end
    end
    return gestures
end

function GestureDetector:deepCopyEv(tev)
    return {
        x = tev.x,
        y = tev.y,
        id = tev.id,
        slot = tev.slot,
        timev = tev.timev, -- No need to make a copy of this one, tev.timev is re-assigned to a new object on every SYN_REPORT
    }
end

--[[
tap2 is the later tap
--]]
function GestureDetector:isTapBounce(tap1, tap2, interval)
    -- NOTE: If time went backwards, make the delta infinite to avoid misdetections,
    --       as we can no longer compute a sensible value...
    local time_diff = tap2.timev - tap1.timev
    if time_diff < 0 then
        time_diff = time.huge
    end
    return (
        math.abs(tap1.x - tap2.x) < self.SINGLE_TAP_BOUNCE_DISTANCE and
        math.abs(tap1.y - tap2.y) < self.SINGLE_TAP_BOUNCE_DISTANCE and
        time_diff < interval
    )
end

function GestureDetector:isDoubleTap(tap1, tap2)
    local time_diff = tap2.timev - tap1.timev
    if time_diff < 0 then
        time_diff = time.huge
    end
    return (
        math.abs(tap1.x - tap2.x) < self.DOUBLE_TAP_DISTANCE and
        math.abs(tap1.y - tap2.y) < self.DOUBLE_TAP_DISTANCE and
        time_diff < self.ges_double_tap_interval
    )
end

-- Takes times as input, not a tev
function GestureDetector:isHold(time1, time2)
    local time_diff = time2 - time1
    if time_diff < 0 then
        time_diff = 0
    end
    -- NOTE: We cheat by not checking a distance because we're only checking that in tapState,
    --       which already ensures a stationary finger, by elimination ;).
    return time_diff >= self.ges_hold_interval
end

function GestureDetector:isTwoFingerTap()
    local s1 = self.input.main_finger_slot
    local s2 = self.input.main_finger_slot + 1

    if self.last_tevs[s1] == nil or self.last_tevs[s2] == nil then
        return false
    end
    local x_diff0 = math.abs(self.last_tevs[s1].x - self.first_tevs[s1].x)
    local x_diff1 = math.abs(self.last_tevs[s2].x - self.first_tevs[s2].x)
    local y_diff0 = math.abs(self.last_tevs[s1].y - self.first_tevs[s1].y)
    local y_diff1 = math.abs(self.last_tevs[s2].y - self.first_tevs[s2].y)
    local time_diff0 = self.last_tevs[s1].timev - self.first_tevs[s1].timev
    if time_diff0 < 0 then
        time_diff0 = time.huge
    end
    local time_diff1 = self.last_tevs[s2].timev - self.first_tevs[s2].timev
    if time_diff1 < 0 then
        time_diff1 = time.huge
    end
    return (
        x_diff0 < self.TWO_FINGER_TAP_REGION and
        x_diff1 < self.TWO_FINGER_TAP_REGION and
        y_diff0 < self.TWO_FINGER_TAP_REGION and
        y_diff1 < self.TWO_FINGER_TAP_REGION and
        time_diff0 < self.ges_two_finger_tap_duration and
        time_diff1 < self.ges_two_finger_tap_duration
    )
end

--[[--
Compares `last_pan` with `first_tev` in this slot.

The second boolean argument `simple` results in only four directions if true.

@return (direction, distance) pan direction and distance
--]]
function GestureDetector:getPath(slot, simple, diagonal, first_tev)
    first_tev = first_tev or self.first_tevs

    local x_diff = self.last_tevs[slot].x - first_tev[slot].x
    local y_diff = self.last_tevs[slot].y - first_tev[slot].y
    local direction = nil
    local distance = math.sqrt(x_diff*x_diff + y_diff*y_diff)
    if x_diff ~= 0 or y_diff ~= 0 then
        local v_direction = y_diff < 0 and "north" or "south"
        local h_direction = x_diff < 0 and "west" or "east"
        if (not simple
            and math.abs(y_diff) > 0.577*math.abs(x_diff)
            and math.abs(y_diff) < 1.732*math.abs(x_diff))
           or (simple and diagonal)
        then
            direction = v_direction..h_direction
        elseif (math.abs(x_diff) > math.abs(y_diff)) then
            direction = h_direction
        else
            direction = v_direction
        end
    end
    return direction, distance
end

function GestureDetector:isSwipe(slot)
    if not self.first_tevs[slot] or not self.last_tevs[slot] then return end
    local time_diff = self.last_tevs[slot].timev - self.first_tevs[slot].timev
    if time_diff < 0 then
        time_diff = time.huge
    end
    if time_diff < self.ges_swipe_interval then
        local x_diff = self.last_tevs[slot].x - self.first_tevs[slot].x
        local y_diff = self.last_tevs[slot].y - self.first_tevs[slot].y
        if x_diff ~= 0 or y_diff ~= 0 then
            return true
        end
    end
end

function GestureDetector:getRotate(orig_point, start_point, end_point)
    local a = orig_point:distance(start_point)
    local b = orig_point:distance(end_point)
    local c = start_point:distance(end_point)
    return math.acos((a*a + b*b - c*c)/(2*a*b))*180/math.pi
end

--[[
Warning! this method won't update self.state, you need to do it
in each state method!
--]]
function GestureDetector:switchState(state_new, tev, param)
    --- @todo Do we need to check whether state is valid?    (houqp)
    return self[state_new](self, tev, param)
end

function GestureDetector:clearState(slot)
    self.states[slot] = self.initialState
    self.pending_hold_timer[slot] = nil
    self.detectings[slot] = false
    self.first_tevs[slot] = nil
    self.last_tevs[slot] = nil
    self.multiswipe_directions = {}
    self.multiswipe_type = nil

    -- Also clear any pending hold callbacks on that slot.
    -- (single taps call this, so we can't clear double_tap callbacks without being caught in an obvious catch-22 ;)).
    self.input:clearTimeout(slot, "hold")
end

function GestureDetector:clearStates()
    for k, _ in pairs(self.states) do
        self:clearState(k)
    end
end

function GestureDetector:initialState(tev)
    local slot = tev.slot
    if tev.id then
        -- an event ends
        if tev.id == -1 then
            self.detectings[slot] = false
        else
            self.track_ids[slot] = tev.id
            if tev.x and tev.y then
                -- user starts a new touch motion
                if not self.detectings[slot] then
                    self.detectings[slot] = true
                    -- NOTE: We can't use a simple reference, because tev is actually Input's self.ev_slots[slot],
                    --       and *that* is a fixed reference for a given slot!
                    --       Here, we really want to rememver the *first* tev, so, make a copy of it.
                    self.first_tevs[slot] = self:deepCopyEv(tev)
                    -- default to tap state
                    return self:switchState("tapState", tev)
                end
            end
        end
    end
end

--[[--
Attempts to figure out which clock source tap events are using...
]]
function GestureDetector:probeClockSource(timev)
    -- We'll check if that timestamp is +/- 2.5s away from the three potential clock sources supported by evdev.
    -- We have bigger issues than this if we're parsing events more than 3s late ;).
    local threshold = time.s(2) + time.ms(500)

    -- Start w/ REALTIME, because it's the easiest to detect ;).
    local realtime = time.realtime_coarse()
    -- clock-threshold <= timev <= clock+threshold
    if timev >= realtime - threshold and timev <= realtime + threshold then
        self.clock_id = C.CLOCK_REALTIME
        logger.dbg("GestureDetector:probeClockSource: Touch event timestamps appear to use CLOCK_REALTIME")
        return
    end

    -- Then MONOTONIC, as it's (hopefully) more common than BOOTTIME (and also guaranteed to be an usable clock source)
    local monotonic = time.monotonic_coarse()
    if timev >= monotonic - threshold and timev <= monotonic + threshold then
        self.clock_id = C.CLOCK_MONOTONIC
        logger.dbg("GestureDetector:probeClockSource: Touch event timestamps appear to use CLOCK_MONOTONIC")
        return
    end

    -- Finally, BOOTTIME
    local boottime = time.boottime()
    -- NOTE: It was implemented in Linux 2.6.39, so, reject 0, which would mean it's unsupported...
    if not boottime == 0 and timev >= boottime - threshold and timev <= boottime + threshold then
        self.clock_id = C.CLOCK_BOOTTIME
        logger.dbg("GestureDetector:probeClockSource: Touch event timestamps appear to use CLOCK_BOOTTIME")
        return
    end

    -- If we're here, the detection was inconclusive :/
    self.clock_id = -1
    logger.dbg("GestureDetector:probeClockSource: Touch event clock source detection was inconclusive")
    -- Print all all the gory details in debug mode when this happens...
    logger.dbg("Input frame    :", time.format_time(timev))
    logger.dbg("CLOCK_REALTIME :", time.format_time(realtime))
    logger.dbg("CLOCK_MONOTONIC:", time.format_time(monotonic))
    logger.dbg("CLOCK_BOOTTIME :", time.format_time(boottime))
end

function GestureDetector:getClockSource()
    return self.clock_id
end

function GestureDetector:resetClockSource()
    self.clock_id = nil
end

--[[--
Handles both single and double tap.
--]]
function GestureDetector:tapState(tev)
    -- Attempt to detect the clock source for these events (we reset it on suspend to discriminate MONOTONIC from BOOTTIME).
    if not self.clock_id then
        self:probeClockSource(tev.timev)
    end

    local slot = tev.slot
    logger.dbg("slot", slot, "in tap state...")
    if tev.id == -1 then
        local s1 = self.input.main_finger_slot
        local s2 = self.input.main_finger_slot + 1
        -- end of tap event
        if self.detectings[s1] and self.detectings[s2] then
            if self:isTwoFingerTap() then
                local pos0 = Geom:new{
                    x = self.last_tevs[s1].x,
                    y = self.last_tevs[s1].y,
                    w = 0, h = 0,
                }
                local pos1 = Geom:new{
                    x = self.last_tevs[s2].x,
                    y = self.last_tevs[s2].y,
                    w = 0, h = 0,
                }
                local tap_span = pos0:distance(pos1)
                logger.dbg("two-finger tap detected with span", tap_span)
                self:clearState(s1)
                self:clearState(s2)
                return {
                    ges = "two_finger_tap",
                    pos = pos0:midpoint(pos1),
                    span = tap_span,
                    time = tev.timev,
                }
            else
                self:clearState(slot)
            end
        elseif self.last_tevs[slot] ~= nil then
            -- Normal single tap seems to always go thru here
            -- (the next 'else' might be there for edge cases)
            return self:handleDoubleTap(tev)
        else
            -- last tev in this slot is cleared by last two finger tap
            self:clearState(slot)
            return {
                ges = "tap",
                pos = Geom:new{
                    x = tev.x,
                    y = tev.y,
                    w = 0, h = 0,
                },
                time = tev.timev,
            }
        end
    else
        return self:handleNonTap(tev)
    end
end

function GestureDetector:handleDoubleTap(tev)
    local slot = tev.slot
    local ges_ev = {
        -- default to single tap
        ges = "tap",
        pos = Geom:new{
            x = self.last_tevs[slot].x,
            y = self.last_tevs[slot].y,
            w = 0, h = 0,
        },
        time = tev.timev,
    }
    -- cur_tap is used for double tap detection
    local cur_tap = {
        x = tev.x,
        y = tev.y,
        timev = tev.timev,
    }

    -- Tap interval / bounce detection may be tweaked by widget (i.e. VirtualKeyboard)
    local tap_interval = self.input.tap_interval_override or self.ges_tap_interval
    -- We do tap bounce detection even when double tap is enabled (so, double tap
    -- is triggered when: ges_tap_interval <= delay < ges_double_tap_interval)
    if tap_interval ~= 0 and self.last_taps[slot] ~= nil and self:isTapBounce(self.last_taps[slot], cur_tap, tap_interval) then
        logger.dbg("tap bounce detected in slot", slot, ": ignored")
        -- Simply ignore it, and clear state as this is the end of a touch event
        -- (this doesn't clear self.last_taps[slot], so a 3rd tap can be detected
        -- as a double tap)
        self:clearState(slot)
        return
    end

    if not self.input.disable_double_tap and self.last_taps[slot] ~= nil and
                self:isDoubleTap(self.last_taps[slot], cur_tap) then
        -- it is a double tap
        self:clearState(slot)
        self.input:clearTimeout(slot, "double_tap")
        ges_ev.ges = "double_tap"
        self.last_taps[slot] = nil
        logger.dbg("double tap detected in slot", slot)
        return ges_ev
    end

    -- set current tap to last tap
    self.last_taps[slot] = cur_tap

    if self.input.disable_double_tap then
        -- We can send the event immediately (no need for the
        -- timer stuff needed for double tap support)
        logger.dbg("single tap detected in slot", slot, ges_ev.pos)
        self:clearState(slot)
        return ges_ev
    end

    -- Double tap enabled: we can't send this single tap immediately as it
    -- may be the start of a double tap. We'll send it as a single tap after
    -- a timer if no second tap happened in the double tap delay.
    logger.dbg("set up single/double tap timer")
    -- setTimeout will handle computing the deadline in the least lossy way possible given the platform.
    self.input:setTimeout(slot, "double_tap", function()
        logger.dbg("in single/double tap timer, single tap:", self.last_taps[slot] ~= nil)
        -- double tap will set last_tap to nil so if it is not, then
        -- user has not double-tap'ed: it's a single tap
        if self.last_taps[slot] ~= nil then
            self.last_taps[slot] = nil
            -- we are using closure here
            logger.dbg("single tap detected in slot", slot, ges_ev.pos)
            return ges_ev
        end
    end, tev.timev, self.ges_double_tap_interval)
    -- we are already at the end of touch event
    -- so reset the state
    self:clearState(slot)
end

function GestureDetector:handleNonTap(tev)
    local slot = tev.slot
    if self.states[slot] ~= self.tapState then
        -- switched from other state, probably from initialState
        -- we return nil in this case
        self.states[slot] = self.tapState
        logger.dbg("set up hold timer")
        -- Invalidate previous hold timers on that slot so that the following setTimeout will only react to *this* tapState.
        self.input:clearTimeout(slot, "hold")
        self.pending_hold_timer[slot] = true
        self.input:setTimeout(slot, "hold", function()
            -- If the pending_hold_timer we set on our first switch to tapState on this slot (e.g., first finger down event),
            -- back when the timer was setup, is still relevant (e.g., the slot wasn't run through clearState by a finger up gesture),
            -- then check that we're still in a stationary finger down state (e.g., tapState).
            if self.pending_hold_timer[slot] and self.states[slot] == self.tapState then
                -- That means we can switch to hold
                logger.dbg("hold gesture detected in slot", slot)
                self.pending_hold_timer[slot] = nil
                return self:switchState("holdState", tev, true)
            end
        end, tev.timev, self.ges_hold_interval)
        return {
            ges = "touch",
            pos = Geom:new{
                x = tev.x,
                y = tev.y,
                w = 0, h = 0,
            },
            time = tev.timev,
        }
    else
        -- We're still inside a stream of input events, see if we need to switch to other states.
        if (tev.x and math.abs(tev.x - self.first_tevs[slot].x) >= self.PAN_THRESHOLD) or
        (tev.y and math.abs(tev.y - self.first_tevs[slot].y) >= self.PAN_THRESHOLD) then
            -- If user's finger moved far enough on the X or Y axes, switch to pan state.
            return self:switchState("panState", tev)
        end
    end
end

function GestureDetector:panState(tev)
    local slot = tev.slot
    logger.dbg("slot", slot, "in pan state...")
    if tev.id == -1 then
        -- end of pan, signal swipe gesture if necessary
        if self:isSwipe(slot) then
            local s1 = self.input.main_finger_slot
            local s2 = self.input.main_finger_slot + 1
            if self.detectings[s1] and self.detectings[s2] then
                local ges_ev = self:handleTwoFingerPan(tev)
                self:clearState(s1)
                self:clearState(s2)
                if ges_ev then
                    if ges_ev.ges == "two_finger_pan" then
                        ges_ev.ges = "two_finger_swipe"
                    elseif ges_ev.ges == "inward_pan" then
                        ges_ev.ges = "pinch"
                    elseif ges_ev.ges == "outward_pan" then
                        ges_ev.ges = "spread"
                    end
                    logger.dbg(ges_ev.ges, ges_ev.direction, ges_ev.distance, "detected")
                end
                return ges_ev
            else
                return self:handleSwipe(tev)
            end
        else -- if end of pan is not swipe then it must be pan release.
            return self:handlePanRelease(tev)
        end
    else
        if self.states[slot] ~= self.panState then
            self.states[slot] = self.panState
        end
        return self:handlePan(tev)
    end
end

function GestureDetector:handleSwipe(tev)
    local slot = tev.slot
    local swipe_direction, swipe_distance = self:getPath(slot)
    local start_pos = Geom:new{
        x = self.first_tevs[slot].x,
        y = self.first_tevs[slot].y,
        w = 0, h = 0,
    }
    local ges = "swipe"
    local multiswipe_directions

    if #self.multiswipe_directions > 1 then
        ges = "multiswipe"
        multiswipe_directions = ""
        for k, v in ipairs(self.multiswipe_directions) do
            local sep = ""
            if k > 1 then
                sep = " "
            end
            multiswipe_directions = multiswipe_directions .. sep .. v[1]
        end
        logger.dbg("multiswipe", multiswipe_directions)
    end

    --- @todo dirty hack for some weird devices, replace it with better solution
    if swipe_direction == "west" and DCHANGE_WEST_SWIPE_TO_EAST then
        swipe_direction = "east"
    elseif swipe_direction == "east" and DCHANGE_EAST_SWIPE_TO_WEST then
        swipe_direction = "west"
    end
    logger.dbg("swipe", swipe_direction, swipe_distance, "detected in slot", slot)
    self:clearState(slot)
    return {
        ges = ges,
        -- use first pan tev coordination as swipe start point
        pos = start_pos,
        direction = swipe_direction,
        multiswipe_directions = multiswipe_directions,
        distance = swipe_distance,
        time = tev.timev,
    }
end

function GestureDetector:handlePan(tev)
    local slot = tev.slot
    local s1 = self.input.main_finger_slot
    local s2 = self.input.main_finger_slot + 1
    if self.detectings[s1] and self.detectings[s2] then
        return self:handleTwoFingerPan(tev)
    else
        local pan_direction, pan_distance = self:getPath(slot)
        local pan_ev = {
            ges = "pan",
            relative = {
                -- default to pan 0
                x = 0,
                y = 0,
            },
            pos = nil,
            direction = pan_direction,
            distance = pan_distance,
            time = tev.timev,
        }

        -- regular pan
        pan_ev.relative.x = tev.x - self.first_tevs[slot].x
        pan_ev.relative.y = tev.y - self.first_tevs[slot].y

        pan_ev.pos = Geom:new{
            x = self.last_tevs[slot].x,
            y = self.last_tevs[slot].y,
            w = 0, h = 0,
        }

        local msd_cnt = #self.multiswipe_directions
        local msd_direction_prev = (msd_cnt > 0) and self.multiswipe_directions[msd_cnt][1] or ""
        local prev_ms_ev, fake_first_tev

        if msd_cnt == 0 then
            -- determine whether to initiate a straight or diagonal multiswipe
            self.multiswipe_type = "straight"
            if pan_direction ~= "north" and pan_direction ~= "south"
               and pan_direction ~= "east" and pan_direction ~= "west" then
                self.multiswipe_type = "diagonal"
            end
        -- recompute a more accurate direction and distance in a multiswipe context
        elseif msd_cnt > 0 then
            prev_ms_ev = self.multiswipe_directions[msd_cnt][2]
            fake_first_tev = {
                [slot] = {
                    ["x"] = prev_ms_ev.pos.x,
                    ["y"] = prev_ms_ev.pos.y,
                    ["slot"] = slot,
                },
            }
        end

        -- the first time fake_first_tev is nil, so self.first_tevs is automatically used instead
        local msd_direction, msd_distance
        if self.multiswipe_type == "straight" then
            msd_direction, msd_distance = self:getPath(slot, true, false, fake_first_tev)
        else
            msd_direction, msd_distance = self:getPath(slot, true, true, fake_first_tev)
        end

        if msd_distance > self.MULTISWIPE_THRESHOLD then
            local pan_ev_multiswipe = pan_ev
            -- store a copy of pan_ev without rotation adjustment
            -- for multiswipe calculations when rotated
            if self.screen:getTouchRotation() > self.screen.ORIENTATION_PORTRAIT then
                pan_ev_multiswipe = util.tableDeepCopy(pan_ev)
            end
            if msd_direction ~= msd_direction_prev then
                self.multiswipe_directions[msd_cnt+1] = {
                    [1] = msd_direction,
                    [2] = pan_ev_multiswipe,
                }
            -- update ongoing swipe direction to the new maximum
            else
                self.multiswipe_directions[msd_cnt] = {
                    [1] = msd_direction,
                    [2] = pan_ev_multiswipe,
                }
            end
        end

        return pan_ev
    end
end

function GestureDetector:handleTwoFingerPan(tev)
    local s1 = self.input.main_finger_slot
    local s2 = self.input.main_finger_slot + 1
    -- triggering slot
    local tslot = tev.slot
    -- reference slot
    local rslot = tslot == s2 and s1 or s2
    local tpan_dir, tpan_dis = self:getPath(tslot)
    local tstart_pos = Geom:new{
        x = self.first_tevs[tslot].x,
        y = self.first_tevs[tslot].y,
        w = 0, h = 0,
    }
    local tend_pos = Geom:new{
        x = self.last_tevs[tslot].x,
        y = self.last_tevs[tslot].y,
        w = 0, h = 0,
    }
    local rstart_pos = Geom:new{
        x = self.first_tevs[rslot].x,
        y = self.first_tevs[rslot].y,
        w = 0, h = 0,
    }
    if self.states[rslot] == self.panState then
        local rpan_dir, rpan_dis = self:getPath(rslot)
        local rend_pos = Geom:new{
            x = self.last_tevs[rslot].x,
            y = self.last_tevs[rslot].y,
            w = 0, h = 0,
        }
        local start_distance = tstart_pos:distance(rstart_pos)
        local end_distance = tend_pos:distance(rend_pos)
        local ges_ev = {
            ges = "two_finger_pan",
            -- use midpoint of tstart and rstart as swipe start point
            pos = tstart_pos:midpoint(rstart_pos),
            distance = tpan_dis + rpan_dis,
            direction = tpan_dir,
            time = tev.timev,
        }
        if tpan_dir ~= rpan_dir then
            if start_distance > end_distance then
                ges_ev.ges = "inward_pan"
            else
                ges_ev.ges = "outward_pan"
            end
            ges_ev.direction = self.DIRECTION_TABLE[tpan_dir]
        end
        logger.dbg(ges_ev.ges, ges_ev.direction, ges_ev.distance, "detected")
        return ges_ev
    elseif self.states[rslot] == self.holdState then
        local angle = self:getRotate(rstart_pos, tstart_pos, tend_pos)
        logger.dbg("rotate", angle, "detected")
        return {
            ges = "rotate",
            pos = rstart_pos,
            angle = angle,
            time = tev.timev,
        }
    end
end

function GestureDetector:handlePanRelease(tev)
    local slot = tev.slot
    local release_pos = Geom:new{
        x = self.last_tevs[slot].x,
        y = self.last_tevs[slot].y,
        w = 0, h = 0,
    }
    local pan_ev = {
        ges = "pan_release",
        pos = release_pos,
        time = tev.timev,
    }
    local s1 = self.input.main_finger_slot
    local s2 = self.input.main_finger_slot + 1
    if self.detectings[s1] and self.detectings[s2] then
        logger.dbg("two finger pan release detected")
        pan_ev.ges = "two_finger_pan_release"
        self:clearState(s1)
        self:clearState(s2)
    else
        logger.dbg("pan release detected in slot", slot)
        self:clearState(slot)
    end
    return pan_ev
end

function GestureDetector:holdState(tev, hold)
    local slot = tev.slot
    logger.dbg("slot", slot, "in hold state...")
    -- When we switch to hold state, we pass an additional boolean param "hold".
    if tev.id ~= -1 and hold and self.last_tevs[slot].x and self.last_tevs[slot].y then
        self.states[slot] = self.holdState
        return {
            ges = "hold",
            pos = Geom:new{
                x = self.last_tevs[slot].x,
                y = self.last_tevs[slot].y,
                w = 0, h = 0,
            },
            time = tev.timev,
        }
    elseif tev.id == -1 and self.last_tevs[slot] ~= nil then
        -- end of hold, signal hold release
        logger.dbg("hold_release detected in slot", slot)
        local last_x = self.last_tevs[slot].x
        local last_y = self.last_tevs[slot].y
        self:clearState(slot)
        return {
            ges = "hold_release",
            pos = Geom:new{
                x = last_x,
                y = last_y,
                w = 0, h = 0,
            },
            time = tev.timev,
        }
    elseif (tev.x and math.abs(tev.x - self.first_tevs[slot].x) >= self.PAN_THRESHOLD) or
        (tev.y and math.abs(tev.y - self.first_tevs[slot].y) >= self.PAN_THRESHOLD) then
        local ges_ev = self:handlePan(tev)
        if ges_ev ~= nil then ges_ev.ges = "hold_pan" end
        return ges_ev
    end
end

local ges_coordinate_translation_270 = {
    north = "west",
    south = "east",
    east = "north",
    west = "south",
    northeast = "northwest",
    northwest = "southwest",
    southeast = "northeast",
    southwest = "southeast",
}
local ges_coordinate_translation_180 = {
    north = "south",
    south = "north",
    east = "west",
    west = "east",
    northeast = "southwest",
    northwest = "southeast",
    southeast = "northwest",
    southwest = "northeast",
}
local ges_coordinate_translation_90 = {
    north = "east",
    south = "west",
    east = "south",
    west = "north",
    northeast = "southeast",
    northwest = "northeast",
    southeast = "southwest",
    southwest = "northwest",
}
local function translateGesDirCoordinate(direction, translation_table)
    return translation_table[direction]
end
local function translateMultiswipeGesDirCoordinate(multiswipe_directions, translation_table)
    return multiswipe_directions:gsub("%S+", translation_table)
end

--[[--
  Changes gesture's `x` and `y` coordinates according to screen view mode.

  @param ges gesture that you want to adjust
  @return adjusted gesture.
--]]
function GestureDetector:adjustGesCoordinate(ges)
    local mode = self.screen:getTouchRotation()
    if mode == self.screen.ORIENTATION_LANDSCAPE then
        -- in landscape mode rotated 90
        if ges.pos then
            ges.pos.x, ges.pos.y = (self.screen:getWidth() - ges.pos.y), (ges.pos.x)
        end
        if ges.ges == "swipe" or ges.ges == "pan"
            or ges.ges == "multiswipe"
            or ges.ges == "two_finger_swipe"
            or ges.ges == "two_finger_pan"
        then
            ges.direction = translateGesDirCoordinate(ges.direction, ges_coordinate_translation_90)
            if ges.ges == "multiswipe" then
                ges.multiswipe_directions = translateMultiswipeGesDirCoordinate(ges.multiswipe_directions, ges_coordinate_translation_90)
            end
            if ges.relative then
                ges.relative.x, ges.relative.y = -ges.relative.y, ges.relative.x
            end
        elseif ges.ges == "pinch" or ges.ges == "spread"
            or ges.ges == "inward_pan"
            or ges.ges == "outward_pan" then
            if ges.direction == "horizontal" then
                ges.direction = "vertical"
            elseif ges.direction == "vertical" then
                ges.direction = "horizontal"
            end
        end
    elseif mode == self.screen.ORIENTATION_LANDSCAPE_ROTATED then
        -- in landscape mode rotated 270
        if ges.pos then
            ges.pos.x, ges.pos.y = (ges.pos.y), (self.screen:getHeight() - ges.pos.x)
        end
        if ges.ges == "swipe" or ges.ges == "pan"
            or ges.ges == "multiswipe"
            or ges.ges == "two_finger_swipe"
            or ges.ges == "two_finger_pan"
        then
            ges.direction = translateGesDirCoordinate(ges.direction, ges_coordinate_translation_270)
            if ges.ges == "multiswipe" then
                ges.multiswipe_directions = translateMultiswipeGesDirCoordinate(ges.multiswipe_directions, ges_coordinate_translation_270)
            end
            if ges.relative then
                ges.relative.x, ges.relative.y = ges.relative.y, -ges.relative.x
            end
        elseif ges.ges == "pinch" or ges.ges == "spread"
            or ges.ges == "inward_pan"
            or ges.ges == "outward_pan" then
            if ges.direction == "horizontal" then
                ges.direction = "vertical"
            elseif ges.direction == "vertical" then
                ges.direction = "horizontal"
            end
        end
    elseif mode == self.screen.ORIENTATION_PORTRAIT_ROTATED then
        -- in portrait mode rotated 180
        if ges.pos then
            ges.pos.x, ges.pos.y = (self.screen:getWidth() - ges.pos.x), (self.screen:getHeight() - ges.pos.y)
        end
        if ges.ges == "swipe" or ges.ges == "pan"
            or ges.ges == "multiswipe"
            or ges.ges == "two_finger_swipe"
            or ges.ges == "two_finger_pan"
        then
            ges.direction = translateGesDirCoordinate(ges.direction, ges_coordinate_translation_180)
            if ges.ges == "multiswipe" then
                ges.multiswipe_directions = translateMultiswipeGesDirCoordinate(ges.multiswipe_directions, ges_coordinate_translation_180)
            end
            if ges.relative then
                ges.relative.x, ges.relative.y = -ges.relative.x, -ges.relative.y
            end
        elseif ges.ges == "pinch" or ges.ges == "spread"
                or ges.ges == "inward_pan"
                or ges.ges == "outward_pan" then
            if ges.direction == "horizontal" then
                ges.direction = "horizontal"
            elseif ges.direction == "vertical" then
                ges.direction = "vertical"
            end
        end
    end
    logger.dbg("adjusted ges:", ges.ges, ges.multiswipe_directions or ges.direction)
    return ges
end

return GestureDetector