doc: pretty-formats note wide char limitations, and add tests

The previous commits added clarifications to the column alignment
placeholders, note that the spaces are optional around the parameters.

Also, a proposed extension [1] to allow hard truncation (without
ellipsis '..') highlighted that the existing code does not play well
with wide characters, such as Asian fonts and emojis.

For example, N wide characters take 2N columns so won't fit an odd number
column width, causing misalignment somewhere.

Further analysis also showed that decomposed characters, e.g. separate
`a` + `umlaut` Unicode code-points may also be mis-counted, in some cases
leaving multiple loose `umlauts` all combined together.

Add some notes about these limitations, and add basic tests to demonstrate
them.

The chosen solution for the tests is to substitute any wide character
that overlaps a splitting boundary for the unicode vertical ellipsis
code point as a rare but 'obvious' substitution.

An alternative could be the substitution with a single dot '.' which
matches regular expression usage, and our two dot ellipsis, and further
in scenarios where the bulk of the text is wide characters, would be
obvious. In mainly 'ascii' scenarios a singleton emoji being substituted
by a dot could be confusing.

It is enough that the tests fail cleanly. The final choice for the
substitute character can be deferred.

[1]
https://lore.kernel.org/git/20221030185614.3842-1-philipoakley@iee.email/

Signed-off-by: Philip Oakley <philipoakley@iee.email>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index e51f1e54e1b4835779dd8cd3afb013c22a7c26e4..3b713344597090037bd6cd7bc034aaeed8316656 100644 (file)
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -157,6 +157,11 @@ The placeholders are:
                                  only works correctly with N >= 2.
                                  Note 2: spaces around the N and M (see below)
                                  values are optional.
+                                 Note 3: Emojis and other wide characters
+                                 will take two display columns, which may
+                                 over-run column boundaries.
+                                 Note 4: decomposed character combining marks
+                                 may be misplaced at padding boundaries.
 '%<|( <M> )':: make the next placeholder take at least until Mth
             display column, padding spaces on the right if necessary.
             Use negative M values for column positions measured

diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 0404491d6ee9fa275764497f917d90a7334d6776..2cba0e0c561b0082324872a50277689b5f2406fd 100755 (executable)
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -1018,4 +1018,31 @@ test_expect_success '%(describe:abbrev=...) vs git describe --abbrev=...' '
        test_cmp expect actual
 '
 
+# pretty-formats note wide char limitations, and add tests
+test_expect_failure 'wide and decomposed characters column counting' '
+
+# from t/lib-unicode-nfc-nfd.sh hex values converted to octal
+       utf8_nfc=$(printf "\303\251") && # e acute combined.
+       utf8_nfd=$(printf "\145\314\201") && # e with a combining acute (i.e. decomposed)
+       utf8_emoji=$(printf "\360\237\221\250") &&
+
+# replacement character when requesting a wide char fits in a single display colum.
+# "half wide" alternative could be a plain ASCII dot `.`
+       utf8_vert_ell=$(printf "\342\213\256") &&
+
+# use ${xxx} here!
+       nfc10="${utf8_nfc}${utf8_nfc}${utf8_nfc}${utf8_nfc}${utf8_nfc}${utf8_nfc}${utf8_nfc}${utf8_nfc}${utf8_nfc}${utf8_nfc}" &&
+       nfd10="${utf8_nfd}${utf8_nfd}${utf8_nfd}${utf8_nfd}${utf8_nfd}${utf8_nfd}${utf8_nfd}${utf8_nfd}${utf8_nfd}${utf8_nfd}" &&
+       emoji5="${utf8_emoji}${utf8_emoji}${utf8_emoji}${utf8_emoji}${utf8_emoji}" &&
+# emoji5 uses 10 display columns
+
+       test_commit "abcdefghij" &&
+       test_commit --no-tag "${nfc10}" &&
+       test_commit --no-tag "${nfd10}" &&
+       test_commit --no-tag "${emoji5}" &&
+       printf "${utf8_emoji}..${utf8_emoji}${utf8_vert_ell}\n${utf8_nfd}..${utf8_nfd}${utf8_nfd}\n${utf8_nfc}..${utf8_nfc}${utf8_nfc}\na..ij\n" >expected &&
+       git log --format="%<(5,mtrunc)%s" -4 >actual &&
+       test_cmp expected actual
+'
+
 test_done