From 453809b65c9823e1957ebcc9e496ff4df7722000 Mon Sep 17 00:00:00 2001
From: jyelon <jyelon@gmail.com>
Date: Wed, 6 Apr 2022 15:09:28 -0400
Subject: [PATCH] A small tweak to lua random

---
 luprex/core/cpp/util.cpp           |  8 ++++++++
 luprex/core/cpp/util.hpp           |  3 +++
 luprex/core/cpp/world-accessor.cpp | 11 +++++++----
 3 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/luprex/core/cpp/util.cpp b/luprex/core/cpp/util.cpp
index fdff486a..4bb9eeb5 100644
--- a/luprex/core/cpp/util.cpp
+++ b/luprex/core/cpp/util.cpp
@@ -157,6 +157,10 @@ uint64_t hash_ints(uint64_t a, uint64_t b, uint64_t c, uint64_t d) {
     return h1;
 }
 
+double hash_to_double(uint64_t hash) {
+    return (hash >> (64-53)) * 0x1p-53;
+}
+
 StringVec split(const eng::string &s, char sep) {
     StringVec result;
     int start = 0;
@@ -498,6 +502,10 @@ LuaDefine(unittests_util, "", "some unit tests") {
     LuaAssertStrEq(L, util::hash_to_hex(util::HashValue(0x1234,0x789a)),
         "0000000000001234000000000000789a");
     
+    // Test hash_to_double
+    LuaAssert(L, util::hash_to_double(0x1000000000000000) == 1.0/16.0);
+    LuaAssert(L, util::hash_to_double(0x7000000000000000) == 7.0/16.0);
+    LuaAssert(L, util::hash_to_double(0xF000000000000000) == 15.0/16.0);
     return 0;
 }
 
diff --git a/luprex/core/cpp/util.hpp b/luprex/core/cpp/util.hpp
index 46bf2961..1a17e559 100644
--- a/luprex/core/cpp/util.hpp
+++ b/luprex/core/cpp/util.hpp
@@ -70,6 +70,9 @@ eng::string hash_to_hex(const HashValue &hash);
 // This is a good hash, but not cryptographically good.
 uint64_t hash_ints(uint64_t n1, uint64_t n2, uint64_t n3, uint64_t n4);
 
+// Convert a 64-bit hash value into a floating point number between 0 and 1.
+double hash_to_double(uint64_t hash);
+
 // Split a string into multiple strings
 StringVec split(const eng::string &s, char sep);
 
diff --git a/luprex/core/cpp/world-accessor.cpp b/luprex/core/cpp/world-accessor.cpp
index f1b4682b..7f58bda7 100644
--- a/luprex/core/cpp/world-accessor.cpp
+++ b/luprex/core/cpp/world-accessor.cpp
@@ -412,12 +412,15 @@ LuaDefine(math_random, "(args...)",
         }
     }
 
-    // Generate the hash and convert to a lua_Number.
-    uint64_t hash = util::hash_ints(seed, count, salt, 456);
     if (!have_range) {
-        double result = (hash & LuaStack::MAXINT) * 0x1p-53;
-        lua_pushnumber(L, result);
+        // Generate the hash and convert to a double.
+        uint64_t hash = util::hash_ints(seed, count, salt, 456);
+        lua_pushnumber(L, util::hash_to_double(hash));
     } else {
+        // Generate the hash and scale it into the desired range.
+        // This code is not quite right: the results are not quite
+        // uniform, this is especially true for very long ranges.
+        uint64_t hash = util::hash_ints(seed, count, salt, 456);
         uint64_t range = (high - low) + 1;
         uint64_t offset = (hash & 0x7FFFFFFFFFFFFFFF) % range;
         int64_t result = low + int64_t(offset);