Initial hashing support

The performance can be improved but this already works.

src/decimal.jl

@@ -77,3 +77,26 @@ end
 # sign
 Base.signbit(x::Decimal) = x.s
 
+function isdivisible(x::BigInt, n::Int)
+    return x % n == 0
+end
+
+function Base.decompose(x::Decimal)
+    if iszero(x)
+        return (big(0), 0, big((-1)^x.s))
+    end
+
+    coef = (-1)^x.s * x.c
+
+    if x.q ≥ 0
+        return (coef * big(5)^x.q, x.q, big(1))
+    else
+        q = -x.q
+        while q > 0 && isdivisible(coef, 5)
+            coef = coef ÷ 5
+            q -= 1
+        end
+        return (coef, x.q, big(5) ^ q)
+    end
+end
+

test/test_decimal.jl

@@ -80,4 +80,22 @@ end
     @test string(number(Decimal(false, 543, -1))) == "54.3"
 end
 
+@testset "Hashing" begin
+    @test hash(Decimal(0.)) == hash(0.)
+    @test hash(Decimal(-0.)) == hash(-0.)
+    @test hash(Decimal(3)) == hash(3)
+    @test hash(Decimal(0.09375)) == hash(0.09375)
+    @test hash(Decimal(-3)) == hash(-3)
+    @test hash(Decimal(-0.09375)) == hash(-0.09375)
+
+    # Equality implies same hash
+    @test hash(Decimal(0, 100, 0)) == hash(Decimal(0, 10, 1))
+    @test hash(Decimal(0, 100, 0)) == hash(Decimal(0, 1, 2))
+    @test hash(Decimal(1, 100, 0)) == hash(Decimal(1, 10, 1))
+    @test hash(Decimal(1, 100, 0)) == hash(Decimal(1, 1, 2))
+
+    @test hash(Decimal(0, 310, -2)) == hash(Decimal(0, 31, -1))
 end
+
+end
+