From 55185ab6aa6f976308ed09d8e60ad51654c3e882 Mon Sep 17 00:00:00 2001
From: pasky <pasky@openttd.org>
Date: Sun, 6 Mar 2005 23:21:57 +0000
Subject: [PATCH] (svn r1951) Introduced SeedModChance() (which is like
 SeedChance() but uses simple modulo instead of bitshifting and
 multiplication), explained why does it work better, used it in
 MakeCzechTownName() and added a TODO note about possibly using it in the
 other town name generators too.

---
 namegen.c | 30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/namegen.c b/namegen.c
index 660d7dee28..8f00406878 100644
--- a/namegen.c
+++ b/namegen.c
@@ -9,6 +9,20 @@ static inline uint32 SeedChance(int shift_by, int max, uint32 seed)
 	return ((uint16)(seed >> shift_by) * max) >> 16;
 }
 
+static inline uint32 SeedModChance(int shift_by, int max, uint32 seed)
+{
+	/* This actually gives *MUCH* more even distribution of the values
+	 * than SeedChance(), which is absolutely horrible in that. If
+	 * you do not believe me, try with i.e. the Czech town names,
+	 * compare the words (nicely visible on prefixes) generated by
+	 * SeedChance() and SeedModChance(). Do not get dicouraged by the
+	 * never-use-modulo myths, which hold true only for the linear
+	 * congruential generators (and Random() isn't such a generator).
+	 * --pasky */
+	// TODO: Perhaps we should use it for all the name generators? --pasky
+	return (seed >> shift_by) % max;
+}
+
 static inline int32 SeedChanceBias(int shift_by, int max, uint32 seed, int bias)
 {
 	return SeedChance(shift_by, max + bias, seed) - bias;
@@ -330,22 +344,22 @@ static byte MakeCzechTownName(char *buf, uint32 seed)
 	enum CzechAllow allow;
 
 	// 1:3 chance to use a real name.
-	if (SeedChance(0, 4, seed) == 0) {
-		strcpy(buf, name_czech_real[SeedChance(1, lengthof(name_czech_real), seed)]);
+	if (SeedModChance(0, 4, seed) == 0) {
+		strcpy(buf, name_czech_real[SeedModChance(4, lengthof(name_czech_real), seed)]);
 		return 0;
 	}
 
 	// NUL terminates the string for strcat()
 	strcpy(buf, "");
 
-	prob_tails = SeedChance(2, 32, seed);
+	prob_tails = SeedModChance(2, 32, seed);
 	do_prefix = prob_tails < 12;
 	do_suffix = prob_tails > 11 && prob_tails < 17;
 
-	if (do_prefix) prefix = SeedChance(5, lengthof(name_czech_adj), seed);
-	if (do_suffix) suffix = SeedChance(7, lengthof(name_czech_suffix), seed);
+	if (do_prefix) prefix = SeedModChance(5, lengthof(name_czech_adj) * 12, seed) / 12;
+	if (do_suffix) suffix = SeedModChance(7, lengthof(name_czech_suffix), seed);
 	// 3:1 chance 3:1 to use dynamic substantive
-	stem = SeedChance(9, lengthof(name_czech_subst_full)
+	stem = SeedModChance(9, lengthof(name_czech_subst_full)
 	                     + 3 * lengthof(name_czech_subst_stem),
 	                   seed);
 	if (stem < (int) lengthof(name_czech_subst_full)) {
@@ -369,7 +383,7 @@ static byte MakeCzechTownName(char *buf, uint32 seed)
 		allow = name_czech_subst_stem[stem].allow;
 
 		// Load the postfix (1:1 chance that a postfix will be inserted)
-		postfix = SeedChance(14, lengthof(name_czech_subst_postfix) * 2, seed);
+		postfix = SeedModChance(14, lengthof(name_czech_subst_postfix) * 2, seed);
 
 		if (choose & CZC_POSTFIX) {
 			// Always get a real postfix.
@@ -415,7 +429,7 @@ static byte MakeCzechTownName(char *buf, uint32 seed)
 		assert(i > 0);
 
 		// Load the ending
-		ending = map[SeedChance(16, i, seed)];
+		ending = map[SeedModChance(16, i, seed)];
 		// Override possible CZG_*FREE; this must be a real gender,
 		// otherwise we get overflow when modifying the adjectivum.
 		gender = name_czech_subst_ending[ending].gender;