make RandIntGen stop handling "full range" random generation

The rand(T) function is already the normal way of generating random values
spanning "full range" of T. So let's make RandIntGen less powerful in the
name of orthogonality and simplicity.

Also tried to clarify a bit the `maxmultiple` comments.

base/random.jl

@@ -148,21 +148,17 @@ rand{T<:Number}(::Type{T}, dims::Int...) = rand(T, dims)
 
 ## Generate random integer within a range 1:n
 
-# remainder function according to Knuth, where rem_knuth(a, 0) = a
-rem_knuth(a::Uint, b::Uint) = a % (b + (b == 0)) + a * (b == 0)
-rem_knuth{T<:Unsigned}(a::T, b::T) = b != 0 ? a % b : a
-
-# maximum multiple of k <= 2^bits(T) decremented by one,
-# that is 0xFFFFFFFF if k = typemax(T) - typemin(T) with intentional underflow
-maxmultiple(k::Uint32) = itrunc(Uint32, div(0x0000000100000000,k + (k == 0))*k - 1)
-maxmultiple(k::Uint64) = itrunc(Uint64, div(0x00000000000000010000000000000000, k + (k == 0))*k - 1)
-# maximum multiple of k within 1:typemax(Uint128)
-maxmultiple(k::Uint128) = div(typemax(Uint128), k + (k == 0))*k - 1
-# maximum multiple of k within 1:2^32 or 1:2^64, depending on size
-maxmultiplemix(k::Uint64) = itrunc(Uint64, div((k >> 32 != 0)*0x0000000000000000FFFFFFFF00000000 + 0x0000000100000000, k + (k == 0))*k - 1)
+# maxmultiple: precondition: k>0
+# maximum multiple of k <= 2^numbits(k), decremented by one
+maxmultiple(k::Uint32) = itrunc(Uint32, div(0x0000000100000000, k)*k - 1)
+maxmultiple(k::Uint64) = itrunc(Uint64, div(0x00000000000000010000000000000000, k)*k - 1)
+# maximum multiple of k <= typemax(Uint128), decremented by one
+maxmultiple(k::Uint128) = div(typemax(Uint128), k)*k - 1
+# 32 or 64 bits version depending on size
+maxmultiplemix(k::Uint64) = k >> 32 == 0 ? uint64(maxmultiple(itrunc(Uint32, k))) : maxmultiple(k)
 
 immutable RandIntGen{T<:Integer, U<:Unsigned}
-    k::U   # range length (or zero for full range)
+    k::U   # range length
     u::U   # rejection threshold
 end
 # generators with 32, 128 bits entropy
@@ -179,7 +175,7 @@ for (T, U) in [(Uint8, Uint32), (Uint16, Uint32),
                (Int8, Uint32), (Int16, Uint32), (Int32, Uint32), (Int64, Uint64), (Int128, Uint128),
                (Bool, Uint32), (Char, Uint32)]
 
-    @eval randintgen(k::$T) = k<1 ? error("range must be non-empty") : RandIntGen($T, convert($U, k)) # overflow ok
+    @eval randintgen(k::$T) = k<1 ? error("range must be non-empty") : RandIntGen($T, convert($U, k))
 end
 
 # this function uses 32 bit entropy for small ranges of length <= typemax(Uint32) + 1
@@ -197,15 +193,15 @@ function rand{T<:Union(Uint64, Int64)}(g::RandIntGen{T,Uint64})
             x = rand(Uint64)
         end
     end
-    return reinterpret(T, one(Uint64) + rem_knuth(x, g.k))
+    return reinterpret(T, one(Uint64) + x % g.k)
 end
 
 function rand{T<:Integer, U<:Unsigned}(g::RandIntGen{T,U})
     x = rand(U)
     while x > g.u
         x = rand(U)
     end
-    itrunc(T, one(U) + rem_knuth(x, g.k))
+    itrunc(T, one(U) + x % g.k)
 end