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Add EUnit tests for seed/2 UTF-32 alignment invariants #9

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Merged
mmzeeman merged 3 commits into from
Apr 10, 2026
Merged

Add EUnit tests for seed/2 UTF-32 alignment invariants #9

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Copilot Apr 10, 2026
3563044
Add seed_test/0 EUnit tests for UTF-32 alignment invariants
Copilot Apr 10, 2026
8317922
Fix misleading comments in seed_test/0
Copilot Apr 10, 2026
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61 changes: 61 additions & 0 deletions src/diffy.erl
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Original file line number Diff line number Diff line change
Expand Up @@ -1486,4 +1486,65 @@ diff_options_test() ->

ok.

seed_test() ->
%% 1. Empty binary: no codepoints, seed is empty.
?assertEqual({0, <<>>}, seed(<<>>, 0)),

%% 2. Binary shorter than 4 codepoints (3 codepoints): 3 div 4 = 0, seed is empty.
Short3 = to_utf32(<<"abc">>),
?assertEqual({0, <<>>}, seed(Short3, 0)),

%% 3. Exactly 4 codepoints, Start=0: seed is 1 codepoint (the first one).
Exact4 = to_utf32(<<"abcd">>),
?assertEqual({0, to_utf32(<<"a">>)}, seed(Exact4, 0)),

%% 4. 8 codepoints, Start=0: seed is 2 codepoints starting at offset 0.
Long8 = to_utf32(<<"12345678">>),
?assertEqual({0, to_utf32(<<"12">>)}, seed(Long8, 0)),

%% 5. 16 codepoints, Start=8 (8 bytes = 2 codepoints * 4 bytes/codepoint):
%% seed is 4 codepoints; returned Start equals 8 and seed bytes are the correct slice.
Long16 = to_utf32(<<"abcdefghijklmnop">>),
{S5, Seed5} = seed(Long16, 8),
?assertEqual(8, S5),
?assertEqual(to_utf32(<<"cdef">>), Seed5),

%% 6. ASCII text round-trip: "1234567890" (10 chars), seed at quarter-way offset.
Ascii10 = to_utf32(<<"1234567890">>),
%% TotalCodepoints=10, SeedCodepoints=2; Start=0 to keep the offset 4-byte-aligned.
{_, SeedAscii} = seed(Ascii10, 0),
?assertEqual(<<"12">>, to_utf8(SeedAscii)),

%% 7. Multi-byte codepoint alignment: 10 Greek letters (2 UTF-8 bytes each, 4 UTF-32 bytes each).
Greek10 = to_utf32(<<"αβγδεζηθικ"/utf8>>),
{Start7, Seed7} = seed(Greek10, 0),
%% Returned Start is 0.
?assertEqual(0, Start7),
%% Seed is 4-byte-aligned.
?assertEqual(0, byte_size(Seed7) rem 4),
%% Seed length = (10 div 4) * 4 = 8 bytes = 2 codepoints.
?assertEqual((10 div 4) * 4, byte_size(Seed7)),
%% Seed decodes back to the first 2 Greek letters.
?assertEqual(<<"αβ"/utf8>>, to_utf8(Seed7)),

%% 8. Emoji (4-byte UTF-8 codepoints): 10 emoji, seed is first 2.
Emoji10 = to_utf32(<<"🐶🐱🐭🐹🐰🐨🐯🦁🐮🐷"/utf8>>),
{_, SeedEmoji} = seed(Emoji10, 0),
%% Seed length = (10 div 4) * 4 = 8 bytes = 2 codepoints.
?assertEqual((10 div 4) * 4, byte_size(SeedEmoji)),
%% Seed decodes back to the first 2 emoji.
?assertEqual(<<"🐶🐱"/utf8>>, to_utf8(SeedEmoji)),

%% 9. Seed start offset preserved: non-zero Start is returned unchanged.
Long12 = to_utf32(<<"abcdefghijkl">>),
{Start9, _} = seed(Long12, 8),
?assertEqual(8, Start9),

%% 10. Seed is a contiguous slice of Long: binary:part(Long, Start, byte_size(Seed)) =:= Seed.
Long20 = to_utf32(<<"abcdefghijklmnopqrst">>),
{Start10, Seed10} = seed(Long20, 8),
?assertEqual(Seed10, binary:part(Long20, Start10, byte_size(Seed10))),

ok.

-endif.