# HG changeset patch # User Mads Kiilerich <mad...@unity3d.com> # Date 1479321935 -3600 # Wed Nov 16 19:45:35 2016 +0100 # Node ID 7f39bccc1c96bffc83f3c6e774da57ecd38982a7 # Parent fe6a3576b863955a6c40ca46bd1d6c8e5384dedf bdiff: early pruning of common prefix before doing expensive computations
It seems quite common that files don't change completely. New lines are often pretty much appended, and modifications will often only change a small section of the file which on average will be in the middle. There can thus be a big win by pruning a common prefix before starting the more expensive search for longest common substrings. Worst case, it will scan through a long sequence of similar bytes without encountering a newline. Splitlines will then have to do the same again ... twice for each side. If similar lines are found, splitlines will save the double iteration and hashing of the lines ... plus there will be less lines to find common substrings in. This change might in some cases make the algorith pick shorter or less optimal common substrings. We can't have the cake and eat it. This make hg --time bundle --base null -r 4.0 go from 14.5 to 15 s - a 3% increase. On mozilla-unified: perfbdiff -m 3041e4d59df2 ! wall 0.053088 comb 0.060000 user 0.060000 sys 0.000000 (best of 100) to ! wall 0.024618 comb 0.020000 user 0.020000 sys 0.000000 (best of 116) perfbdiff 0e9928989e9c --alldata --count 10 ! wall 0.702075 comb 0.700000 user 0.700000 sys 0.000000 (best of 15) to ! wall 0.579235 comb 0.580000 user 0.580000 sys 0.000000 (best of 18) diff --git a/mercurial/bdiff_module.c b/mercurial/bdiff_module.c --- a/mercurial/bdiff_module.c +++ b/mercurial/bdiff_module.c @@ -61,12 +61,12 @@ nomem: static PyObject *bdiff(PyObject *self, PyObject *args) { - char *sa, *sb, *rb; + char *sa, *sb, *rb, *ia, *ib; PyObject *result = NULL; struct bdiff_line *al, *bl; struct bdiff_hunk l, *h; int an, bn, count; - Py_ssize_t len = 0, la, lb; + Py_ssize_t len = 0, la, lb, li = 0, lcommon = 0, lmax; PyThreadState *_save; l.next = NULL; @@ -80,8 +80,17 @@ static PyObject *bdiff(PyObject *self, P } _save = PyEval_SaveThread(); - an = bdiff_splitlines(sa, la, &al); - bn = bdiff_splitlines(sb, lb, &bl); + + lmax = la > lb ? lb : la; + for (ia = sa, ib = sb; + li < lmax && *ia == *ib; + ++li, ++ia, ++ib) + if (*ia == '\n') + lcommon = li + 1; + /* we can almost add: if (li == lmax) lcommon = li; */ + + an = bdiff_splitlines(sa + lcommon, la - lcommon, &al); + bn = bdiff_splitlines(sb + lcommon, lb - lcommon, &bl); if (!al || !bl) goto nomem; @@ -112,8 +121,8 @@ static PyObject *bdiff(PyObject *self, P for (h = l.next; h; h = h->next) { if (h->a1 != la || h->b1 != lb) { len = bl[h->b1].l - bl[lb].l; - putbe32((uint32_t)(al[la].l - al->l), rb); - putbe32((uint32_t)(al[h->a1].l - al->l), rb + 4); + putbe32((uint32_t)(al[la].l + lcommon - al->l), rb); + putbe32((uint32_t)(al[h->a1].l + lcommon - al->l), rb + 4); putbe32((uint32_t)len, rb + 8); memcpy(rb + 12, bl[lb].l, len); rb += 12 + len; diff --git a/tests/test-bdiff.py.out b/tests/test-bdiff.py.out --- a/tests/test-bdiff.py.out +++ b/tests/test-bdiff.py.out @@ -34,10 +34,9 @@ showdiff( showdiff( 'a\nb\nb\nb\nc\n.\nd\ne\n.\nf\n', 'a\nb\nb\na\nb\nb\nb\nc\n.\nb\nc\n.\nd\ne\nf\n'): - 0 0 '' -> 'a\nb\nb\n' - 'a\nb\nb\nb\nc\n.\n' - 12 12 '' -> 'b\nc\n.\n' - 'd\ne\n' + 'a\nb\nb\n' + 6 6 '' -> 'a\nb\nb\nb\nc\n.\n' + 'b\nc\n.\nd\ne\n' 16 18 '.\n' -> '' 'f\n' done diff --git a/tests/test-check-code.t b/tests/test-check-code.t --- a/tests/test-check-code.t +++ b/tests/test-check-code.t @@ -15,6 +15,10 @@ New errors are not allowed. Warnings are Skipping hgext/fsmonitor/pywatchman/msc_stdint.h it has no-che?k-code (glob) Skipping hgext/fsmonitor/pywatchman/pybser.py it has no-che?k-code (glob) Skipping i18n/polib.py it has no-che?k-code (glob) + mercurial/bdiff_module.c:89: + > //printf("common prefix: %i next line: %i\n", li, llf); + don't use //-style comments Skipping mercurial/httpclient/__init__.py it has no-che?k-code (glob) Skipping mercurial/httpclient/_readers.py it has no-che?k-code (glob) Skipping mercurial/statprof.py it has no-che?k-code (glob) + [1] _______________________________________________ Mercurial-devel mailing list Mercurial-devel@mercurial-scm.org https://www.mercurial-scm.org/mailman/listinfo/mercurial-devel