Added composition

Author: scheinerman

Project.toml

@@ -1,6 +1,6 @@
 name = "Bijections"
 uuid = "e2ed5e7c-b2de-5872-ae92-c73ca462fb04"
-version = "0.1.5"
+version = "0.1.6"
 
 [compat]
 julia = "1"

README.md

@@ -218,3 +218,21 @@ contains no pairs:
 julia> isempty(b)
 false
 ```
+
+
+## Composition
+
+Given two `Bijection`s `a` and `b`, their composition `c = a*b` is a new 
+`Bijection` with the property that `c[x] = a[b[x]]` for all `x` in the 
+domain of `b`.
+
+```
+julia> a = Bijection{Int,Int}(); a[1] = 10; a[2] = 20;
+
+julia> b = Bijection{String,Int}(); b["hi"] = 1; b["bye"] = 2;
+
+julia> c = a * b;
+
+julia> c["hi"]
+10
+```

src/Bijections.jl

@@ -199,5 +199,6 @@ function get(b::Bijection, key, default)
 end
 
 include("inversion.jl")
+include("composition.jl")
 
 end # end of module Bijections

src/composition.jl

@@ -0,0 +1,22 @@
+import Base.(*)
+
+"""
+    (*)(a::Bijection{A,B}, b::Bijection{B,C})::Bijection{A,C} where {A,B,C}
+
+The result of `a * b` is a new `Bijection` `c` such that `c[x]` is `a[b[x]]` for `x`
+in the domain of `b`. This function throws an error is `domain(a)` is not the same 
+as the `image` of `b`.    
+"""
+function (*)(a::Bijection{B,A}, b::Bijection{C,B})::Bijection{C,A} where {A,B,C}
+    # check that the domain of a equals the image of B
+    if domain(a) != image(b)
+        error("Domain/Image mismatch")
+    end
+
+    c = Bijection{C,A}()
+    for x in domain(b)
+        c[x] = a[b[x]]
+    end
+
+    return c
+end

test/runtests.jl

@@ -1,45 +1,65 @@
 using Test
 using Bijections
 
-b = Bijection(3,"Hello")
-@test b[3] == "Hello"
-@test b("Hello") == 3
-@test haskey(b, 3)
-@test !haskey(b, 4)
-@test !haskey(b, "Hello")
+@testset "Basics" begin
+    b = Bijection(3, "Hello")
+    @test b[3] == "Hello"
+    @test b("Hello") == 3
+    @test haskey(b, 3)
+    @test !haskey(b, 4)
+    @test !haskey(b, "Hello")
 
-b[2] = "Bye"
-@test b[2] == "Bye"
-@test b("Bye") == 2
-@test inverse(b,"Bye") == 2
+    b[2] = "Bye"
+    @test b[2] == "Bye"
+    @test b("Bye") == 2
+    @test inverse(b, "Bye") == 2
 
 
-@test domain(b) == Set([2,3])
-@test image(b) == Set(["Bye", "Hello"])
+    @test domain(b) == Set([2, 3])
+    @test image(b) == Set(["Bye", "Hello"])
 
-@test length(b) == 2
-@test !isempty(b)
+    @test length(b) == 2
+    @test !isempty(b)
 
 
-bb = inv(b)
-@test bb["Bye"] == 2
-bb["xx"] = 4
-@test length(bb) != length(b)
+    bb = inv(b)
+    @test bb["Bye"] == 2
+    bb["xx"] = 4
+    @test length(bb) != length(b)
 
-bb = active_inv(b)
-@test bb["Bye"] == 2
-b[0] = "Ciao"
-@test bb["Ciao"] == 0
-@test inv(bb) == b
+    bb = active_inv(b)
+    @test bb["Bye"] == 2
+    b[0] = "Ciao"
+    @test bb["Ciao"] == 0
+    @test inv(bb) == b
 
-# iteration test
-dom_list = [x for (x,y) in b]
-@test Set(dom_list) == domain(b)
 
-# conversion to a Dict
-d = Dict(b)
-@test d[0] == b[0]
-@test Bijection(d) == b
+    # iteration test
+    dom_list = [x for (x, y) in b]
+    @test Set(dom_list) == domain(b)
 
-# check pair constructor
-@test Bijection(collect(b)) == b
+
+    # conversion to a Dict
+    d = Dict(b)
+    @test d[0] == b[0]
+    @test Bijection(d) == b
+
+
+    # check pair constructor
+    @test Bijection(collect(b)) == b
+end
+
+# check composition
+@testset "Composition" begin
+    a = Bijection{Int,Int}()
+    a[1] = 10
+    a[2] = 20
+
+    b = Bijection{String,Int}()
+    b["hi"] = 1
+    b["bye"] = 2
+
+    c = a * b
+    @test c["hi"] == 10
+
+end