added math.randrange() and math.randrangew()

compiler/res/prog8lib/math.p8

@@ -75,7 +75,7 @@ _sinecosR8	.char  trunc(127.0 * sin(range(180+45) * rad(360.0/180.0)))
         }}
     }
 
-    asmsub rnd() -> ubyte @A {
+    asmsub rnd() clobbers(Y) -> ubyte @A {
         %asm {{
             jmp  math.randbyte
         }}
@@ -87,6 +87,24 @@ _sinecosR8	.char  trunc(127.0 * sin(range(180+45) * rad(360.0/180.0)))
         }}
     }
 
+    sub randrange(ubyte n) -> ubyte {
+        ; -- return random number uniformly distributed from 0 to n-1 (compensates for divisibility bias)
+        cx16.r0H = 255 / n * n
+        do {
+            cx16.r0L = math.rnd()
+        } until cx16.r0L < cx16.r0H
+        return cx16.r0L % n
+    }
+
+    sub randrangew(uword n) -> uword {
+        ; -- return random number uniformly distributed from 0 to n-1 (compensates for divisibility bias)
+        cx16.r1 = 65535 / n * n
+        do {
+            cx16.r0 = math.rndw()
+        } until cx16.r0 < cx16.r1
+        return cx16.r0 % n
+    }
+
     asmsub rndseed(uword seed1 @AY, uword seed2 @R0) clobbers(A,Y) {
         ; -- set new pseudo RNG's seed values. Defaults are: $00c2, $1137
         %asm {{

compiler/res/prog8lib/virtual/math.p8

@@ -176,6 +176,24 @@ math {
         }}
     }
 
+    sub randrange(ubyte n) -> ubyte {
+        ; -- return random number uniformly distributed from 0 to n-1 (compensates for divisibility bias)
+        cx16.r0H = 255 / n * n
+        do {
+            cx16.r0L = math.rnd()
+        } until cx16.r0L < cx16.r0H
+        return cx16.r0L % n
+    }
+
+    sub randrangew(uword n) -> uword {
+        ; -- return random number uniformly distributed from 0 to n-1 (compensates for divisibility bias)
+        cx16.r1 = 65535 / n * n
+        do {
+            cx16.r0 = math.rndw()
+        } until cx16.r0 < cx16.r1
+        return cx16.r0 % n
+    }
+
     sub rndseed(uword seed1, uword seed2) {
         ; -- reset the pseudo RNG's seed values. Defaults are: $a55a, $7653.
         %ir {{

docs/source/libraries.rst

@@ -458,6 +458,12 @@ but perhaps the provided ones can be of service too.
 ``rndw ()``
     Returns next random word 0-65535 from the pseudo-RNG sequence.
 
+``randrange (ubyte n) -> ubyte``
+    Returns random byte uniformly distributed from 0 to n-1 (compensates for divisibility bias)
+
+``randrangew (uword n) -> uword``
+    Returns random word uniformly distributed from 0 to n-1 (compensates for divisibility bias)
+
 ``rndseed (uword seed1, uword seed2)``
     Sets a new seed for the pseudo-RNG sequence (both rnd and rndw). The seed consists of two words.
     Do not use zeros for the seed!

examples/test.p8

@@ -1,25 +1,43 @@
 %import textio
-%import string
+%import math
-
 %zeropage basicsafe
 %option no_sysinit
 
 main {
-
     sub start() {
-        screen.text_colors = [6,5,4,3,2,1]
+        uword[7] rolls
 
-        for cx16.r0L in screen.text_colors {
+        txt.print("wait...\n")
+        repeat 30 {
+            repeat 1000 {
+                unroll 10 rolls[math.rnd() % len(rolls)]++
+            }
+        }
+
+        for cx16.r0L in 0 to len(rolls)-1 {
             txt.print_ub(cx16.r0L)
             txt.spc()
+            txt.print_uw(rolls[cx16.r0L])
+            txt.nl()
+        }
+
+        txt.print("wait...\n")
+        rolls = [0,0,0,0,0,0,0]
+        repeat 30 {
+            repeat 1000 {
+                unroll 10 rolls[math.randrange(len(rolls))]++
+            }
+        }
+
+        for cx16.r0L in 0 to len(rolls)-1 {
+            txt.print_ub(cx16.r0L)
+            txt.spc()
+            txt.print_uw(rolls[cx16.r0L])
+            txt.nl()
         }
-        txt.nl()
     }
 }
 
-screen {
-    ubyte[6] text_colors
-}
 
 /*main222 {
     sub start() {