Created Line Based Diffs (markdown)

Line-Based-Diffs.md

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+
+### Myers' diff algorithm
+
+This is the default diff algorithm in GNU diff and git diff. It finds
+the longest common subsequence (LCS) and is used on a line-by-line basis.
+
+There's a
+[great introduction
+here](https://blog.jcoglan.com/2017/02/12/the-myers-diff-algorithm-part-1/)
+and the original paper is [An O(ND) Difference Algorithm and Its
+Variations, Myers
+1986](http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.4.6927).
+
+```
+# Modern diff supports colour, but see also
+# https://www.colordiff.org/
+$ diff --color=always -u sample_files/css_before.css sample_files/css_after.css
+```
+
+Note that GNU diff [originally used the Hunt-McIlroy
+algorithm](http://fabiensanglard.net/git_code_review/diff.php)).
+
+### Patience Diff
+
+Myer's diff has a problem with sliders:
+
+```diff
+ if (!$smtp_server) {
++       $smtp_server = $repo->config('sendemail.smtpserver');
++}
++if (!$smtp_server) {
+        foreach (qw( /usr/sbin/sendmail /usr/lib/sendmail )) {
+                if (-x $_) {
+                        $smtp_server = $_;
+```
+
+Instead of:
+
+```diff
++if (!$smtp_server) {
++       $smtp_server = $repo->config('sendemail.smtpserver');
++}
+ if (!$smtp_server) {
+        foreach (qw( /usr/sbin/sendmail /usr/lib/sendmail )) {
+                if (-x $_) {
+```
+
+Git has a `--indent-heuristic` that [was added to reduce the
+likelihood of making a bad
+choice](https://github.com/git/git/commit/433860f3d0beb0c6f205290bd16cda413148f098). There's
+a [corpus of test files](https://github.com/mhagger/diff-slider-tools)
+where the ideal diff has been chosen by a human, to test different
+heuristics.
+
+The [patience diff
+algorithm](https://bramcohen.livejournal.com/73318.html) is an LCS
+algorithm that aims to do a better job with sliders. It produces great
+results by [doing more 
+work](https://stackoverflow.com/questions/40133534/is-gits-implementation-of-the-patience-diff-algorithm-correct/40159510#40159510).
+
+```
+# Original behaviour
+$ git diff --no-indent-heuristic --no-index sample_files/css_before.css sample_files/css_after.css
+# As of git 2.11, this heuristic is enabled by default.
+$ git diff --indent-heuristic --no-index sample_files/css_before.css sample_files/css_after.css
+# Patience algorithm does a better a job in this example.
+$ git diff --patience --no-index sample_files/css_before.css sample_files/css_after.css
+```
+
+[Diff Match Patch](https://github.com/google/diff-match-patch) also
+has some excellent discussions of diff designs on the author's website
+(e.g [diff strategies](https://neil.fraser.name/writing/diff/)).
+
+### Histogram Diff
+
+Git 1.7.7+ also has a [histogram
+algorithm](https://stackoverflow.com/a/32367597/509706), which aims to
+produce better results than Myers' algorithm but without [the slowdown
+of the patience algorithm](https://github.com/git/git/commit/85551232b56e763ecfcc7222e0858bac4e962c80).
+
+```
+# Inferior to patience on this example file.
+$ git diff --histogram --no-index sample_files/css_before.css sample_files/css_after.css
+```
+
+### Side-by-side Diff
+
+```
+$ diff -y --color=always sample_files/css_before.css sample_files/css_after.css
+```