Merge pull request #23 from oxinabox/ox/construct

Constructing one ShortString from another

Author: xiaodaigh

src/base.jl

@@ -6,13 +6,17 @@ struct ShortString{T} <: AbstractString where T
     size_content::T
 end
 
+# check if a string of size `sz` can be stored in ShortString{T}`
+function check_size(T, sz)
+    max_len = sizeof(T) - size_nibbles(T)  # the last few nibbles are is used to store the length
+    if sz > max_len
+        throw(ErrorException("sizeof(::ShortString) must be shorter than or equal to $(max_len) in length; you have supplied a string of size $sz"))
+    end
+end
 
 function ShortString{T}(s::Union{String, SubString{String}}) where T
     sz = sizeof(s)
-    max_len = sizeof(T) - size_nibbles(T)
-    if sz > max_len # the last byte is used to store the length
-        throw(ErrorException("sizeof(::ShortString) must be shorter than or equal to $(max_len) in length; you have supplied a string of size $sz"))
-    end
+    check_size(T, sz)
     bits_to_wipe = 8(sizeof(T) - sz)
     # TODO some times this can throw errors for longish strings
     # Exception: EXCEPTION_ACCESS_VIOLATION at 0x1e0b7afd -- bswap at C:\Users\RTX2080\.julia\packages\BitIntegers\xU40U\src\BitIntegers.jl:332 [inlined]
@@ -21,6 +25,19 @@ function ShortString{T}(s::Union{String, SubString{String}}) where T
     ShortString{T}(content | T(sz))
 end
 
+ShortString{T}(s::ShortString{T}) where T = s
+function ShortString{T}(s::ShortString{S}) where {T, S}
+    sz = sizeof(s)
+    check_size(T, sz)
+    # Flip it so empty bytes are at start, grow/shrink it, flip it back
+    # S(size_mask(S)) will return a mask for getting the size for Shorting Strings in (content size)
+    # format, so something like 00001111 in binary.
+    #  ~S(size_mask(S))) will yield 11110000 which can be used as maks to extract the content
+    content = ntoh(T(ntoh(s.size_content & ~S(size_mask(S)))))
+    ShortString{T}(content | T(sz))
+end
+
+
 String(s::ShortString) = String(reinterpret(UInt8, [s.size_content|>ntoh])[1:sizeof(s)])
 
 Base.codeunit(s::ShortString) = codeunit(String(s))

test/runtests.jl

@@ -70,3 +70,16 @@ basic_test(ShortString{MyUInt2048}, 254)
     @test cmp(ShortString3("α"), "a") == 1
     @test cmp(ShortString3("b"), "β") == -1
 end
+
+@testset "Construction from other ShortStrings" begin
+    @test ShortString7(ShortString3("ab")) == "ab"
+    @test ShortString7(ShortString3("ab")) isa ShortString7
+
+    @test ShortString3(ShortString7("ab")) == "ab"
+    @test ShortString3(ShortString7("ab")) isa ShortString3
+
+    @test ShortString7(ShortString7("ab")) == "ab"
+    @test ShortString7(ShortString7("ab")) isa ShortString7
+
+    @test_throws ErrorException ShortString3(ShortString7("123456"))
+end