rust/hg-core/src/copy_tracing.rs

copies-rust: use immutable "OrdMap" to store copies information

The large majority of time is currently spent coping and merging directories.
the `IM` crate offer "immutable" Map, that use "copy on write" internally.  The
new object use the same API as the standard HashMap.  So switching to it is
trivial and it reduce copying cost significantly. More importantly, using
immutable structure will unlock new possibility for a massive speed up of the
"merging" part. This will came in a later changesets.

Performance wise, we get very significant boost in the worst case. Below is some
highlight of how we fare compared to the previous changeset.

Repo            Cases                                 Source-Rev   Dest-Rev         Old-Time      New-Time     Difference   Factor
------------------------------------------------------------------------------------------------------------------------------------
pypy            x0000_revs_x_added_0_copies           d1defd0dc478 c9cb1334cc78 :  62.468362 s,  33.527067 s, -28.941295 s, × 0.5367
mozilla-central x000_revs_x000_added_x000_copies      7c97034feb78 4407bd0c6330 :   3.619850 s,   0.963905 s,  -2.655945 s, × 0.2663
mozilla-central x0000_revs_xx000_added_x000_copies    f78c615a656c 96a38b690156 :  11.926587 s,   4.217003 s,  -7.709584 s, × 0.3536
mozilla-try     x0000_revs_x_added_0_copies           63519bfd42ee a36a2a865d92 :  10.674920 s,   1.114864 s,  -9.560056 s, × 0.1044
mozilla-try     x00000_revs_x00000_added_0_copies     dc8a3ca7010e d16fde900c9c :  19.647038 s,   1.442793 s, -18.204245 s, × 0.0734


And we sometimes catch up with the performance of the python code as highlighted
below:

Repo            Cases                                 Source-Rev   Dest-Rev         Py-time       Rust-time    Difference   Factor
------------------------------------------------------------------------------------------------------------------------------------
mozilla-try     x00000_revs_x00000_added_0_copies     dc8a3ca7010e d16fde900c9c :   1.074593 s,   1.442793 s,  +0.368200 s, × 1.3426

However, multiple case remains significantly slower, as highlighted below

Repo            Cases                                 Source-Rev   Dest-Rev         Py-time       Rust-time    Difference   Factor
------------------------------------------------------------------------------------------------------------------------------------
mozilla-central x000_revs_x000_added_x000_copies      7c97034feb78 4407bd0c6330 :   0.190133 s,   0.963905 s,  +0.773772 s, × 5.0696
mozilla-central x0000_revs_xx000_added_x000_copies    f78c615a656c 96a38b690156 :   0.440694 s,   4.217003 s,  +3.776309 s, × 9.5690
mozilla-try     x0000_revs_x_added_0_copies           63519bfd42ee a36a2a865d92 :   0.370675 s,   1.114864 s,  +0.744189 s, × 3.0077
pypy            x0000_revs_x_added_0_copies           d1defd0dc478 c9cb1334cc78 :   3.581556 s,  33.527067 s, +29.945511 s, × 9.3610

Below are two different tables for full performance comparison
- this changeset against the previous one (spoiler: it is much better)
- this changeset against the python code  (spoiler: still slower, but it gets more comparable)

This changeset compared to the previous one
===========================================

Repo            Cases                                 Source-Rev   Dest-Rev         Old-Time      New-Time     Difference   Factor
------------------------------------------------------------------------------------------------------------------------------------
mercurial       x_revs_x_added_0_copies               ad6b123de1c7 39cfcef4f463 :   0.000046 s,   0.000049 s,  +0.000003 s, × 1.0652
mercurial       x_revs_x_added_x_copies               2b1c78674230 0c1d10351869 :   0.000173 s,   0.000179 s,  +0.000006 s, × 1.0347
mercurial       x000_revs_x000_added_x_copies         81f8ff2a9bf2 dd3267698d84 :   0.006303 s,   0.006494 s,  +0.000191 s, × 1.0303
pypy            x_revs_x_added_0_copies               aed021ee8ae8 099ed31b181b :   0.000229 s,   0.000339 s,  +0.000110 s, × 1.4803
pypy            x_revs_x000_added_0_copies            4aa4e1f8e19a 359343b9ac0e :   0.000056 s,   0.000057 s,  +0.000001 s, × 1.0179
pypy            x_revs_x_added_x_copies               ac52eb7bbbb0 72e022663155 :   0.000143 s,   0.000299 s,  +0.000156 s, × 2.0909
pypy            x_revs_x00_added_x_copies             c3b14617fbd7 ace7255d9a26 :   0.001166 s,   0.001200 s,  +0.000034 s, × 1.0292
pypy            x_revs_x000_added_x000_copies         df6f7a526b60 a83dc6a2d56f :   0.022931 s,   0.025120 s,  +0.002189 s, × 1.0955
pypy            x000_revs_xx00_added_0_copies         89a76aede314 2f22446ff07e :   0.852446 s,   0.506921 s,  -0.345525 s, × 0.5947
pypy            x000_revs_x000_added_x_copies         8a3b5bfd266e 2c68e87c3efe :   2.221824 s,   1.272060 s,  -0.949764 s, × 0.5725
pypy            x000_revs_x000_added_x000_copies      89a76aede314 7b3dda341c84 :   1.194162 s,   0.690941 s,  -0.503221 s, × 0.5786
pypy            x0000_revs_x_added_0_copies           d1defd0dc478 c9cb1334cc78 :  62.468362 s,  33.527067 s, -28.941295 s, × 0.5367
pypy            x0000_revs_xx000_added_0_copies       bf2c629d0071 4ffed77c095c :   0.022116 s,   0.021970 s,  -0.000146 s, × 0.9934
pypy            x0000_revs_xx000_added_x000_copies    08ea3258278e d9fa043f30c0 :   2.972788 s,   1.772094 s,  -1.200694 s, × 0.5961
netbeans        x_revs_x_added_0_copies               fb0955ffcbcd a01e9239f9e7 :   0.000180 s,   0.000185 s,  +0.000005 s, × 1.0278
netbeans        x_revs_x000_added_0_copies            6f360122949f 20eb231cc7d0 :   0.000123 s,   0.000135 s,  +0.000012 s, × 1.0976
netbeans        x_revs_x_added_x_copies               1ada3faf6fb6 5a39d12eecf4 :   0.000315 s,   0.000329 s,  +0.000014 s, × 1.0444
netbeans        x_revs_x00_added_x_copies             35be93ba1e2c 9eec5e90c05f :   0.001297 s,   0.001343 s,  +0.000046 s, × 1.0355
netbeans        x000_revs_xx00_added_0_copies         eac3045b4fdd 51d4ae7f1290 :   0.024884 s,   0.029396 s,  +0.004512 s, × 1.1813
netbeans        x000_revs_x000_added_x_copies         e2063d266acd 6081d72689dc :   0.032653 s,   0.040210 s,  +0.007557 s, × 1.2314
netbeans        x000_revs_x000_added_x000_copies      ff453e9fee32 411350406ec2 :   4.230118 s,   4.556794 s,  +0.326676 s, × 1.0772
mozilla-central x_revs_x_added_0_copies               3697f962bb7b 7015fcdd43a2 :   0.000197 s,   0.000199 s,  +0.000002 s, × 1.0102
mozilla-central x_revs_x000_added_0_copies            dd390860c6c9 40d0c5bed75d :   0.000622 s,   0.000639 s,  +0.000017 s, × 1.0273
mozilla-central x_revs_x_added_x_copies               8d198483ae3b 14207ffc2b2f :   0.000296 s,   0.000542 s,  +0.000246 s, × 1.8311
mozilla-central x_revs_x00_added_x_copies             98cbc58cc6bc 446a150332c3 :   0.001626 s,   0.001685 s,  +0.000059 s, × 1.0363
mozilla-central x_revs_x000_added_x000_copies         3c684b4b8f68 0a5e72d1b479 :   0.006218 s,   0.006954 s,  +0.000736 s, × 1.1184
mozilla-central x_revs_x0000_added_x0000_copies       effb563bb7e5 c07a39dc4e80 :   0.132760 s,   0.132938 s,  +0.000178 s, × 1.0013
mozilla-central x000_revs_xx00_added_0_copies         6100d773079a 04a55431795e :   0.029001 s,   0.008683 s,  -0.020318 s, × 0.2994
mozilla-central x000_revs_x000_added_x_copies         9f17a6fc04f9 2d37b966abed :   0.005886 s,   0.005956 s,  +0.000070 s, × 1.0119
mozilla-central x000_revs_x000_added_x000_copies      7c97034feb78 4407bd0c6330 :   3.619850 s,   0.963905 s,  -2.655945 s, × 0.2663
mozilla-central x0000_revs_xx000_added_0_copies       9eec5917337d 67118cc6dcad :   0.058678 s,   0.049239 s,  -0.009439 s, × 0.8391
mozilla-central x0000_revs_xx000_added_x000_copies    f78c615a656c 96a38b690156 :  11.926587 s,   4.217003 s,  -7.709584 s, × 0.3536
mozilla-try     x_revs_x_added_0_copies               aaf6dde0deb8 9790f499805a :   0.001204 s,   0.001197 s,  -0.000007 s, × 0.9942
mozilla-try     x_revs_x000_added_0_copies            d8d0222927b4 5bb8ce8c7450 :   0.001217 s,   0.001213 s,  -0.000004 s, × 0.9967
mozilla-try     x_revs_x_added_x_copies               092fcca11bdb 936255a0384a :   0.000605 s,   0.000762 s,  +0.000157 s, × 1.2595
mozilla-try     x_revs_x00_added_x_copies             b53d2fadbdb5 017afae788ec :   0.001876 s,   0.001909 s,  +0.000033 s, × 1.0176
mozilla-try     x_revs_x000_added_x000_copies         20408ad61ce5 6f0ee96e21ad :   0.078190 s,   0.093021 s,  +0.014831 s, × 1.1897
mozilla-try     x_revs_x0000_added_x0000_copies       effb563bb7e5 c07a39dc4e80 :   0.135428 s,   0.134536 s,  -0.000892 s, × 0.9934
mozilla-try     x000_revs_xx00_added_0_copies         6100d773079a 04a55431795e :   0.029123 s,   0.009071 s,  -0.020052 s, × 0.3115
mozilla-try     x000_revs_x000_added_x_copies         9f17a6fc04f9 2d37b966abed :   0.006141 s,   0.006206 s,  +0.000065 s, × 1.0106
mozilla-try     x000_revs_x000_added_x000_copies      1346fd0130e4 4c65cbdabc1f :   4.857827 s,   1.150502 s,  -3.707325 s, × 0.2368
mozilla-try     x0000_revs_x_added_0_copies           63519bfd42ee a36a2a865d92 :  10.674920 s,   1.114864 s,  -9.560056 s, × 0.1044
mozilla-try     x0000_revs_x_added_x_copies           9fe69ff0762d bcabf2a78927 :   9.789462 s,   1.042658 s,  -8.746804 s, × 0.1065
mozilla-try     x0000_revs_xx000_added_x_copies       156f6e2674f2 4d0f2c178e66 :   1.087890 s,   0.447402 s,  -0.640488 s, × 0.4113
mozilla-try     x0000_revs_xx000_added_0_copies       9eec5917337d 67118cc6dcad :   0.060556 s,   0.051132 s,  -0.009424 s, × 0.8444
mozilla-try     x0000_revs_xx000_added_x000_copies    89294cd501d9 7ccb2fc7ccb5 : killed         ,  83.508590 s
mozilla-try     x0000_revs_x0000_added_x0000_copies   e928c65095ed e951f4ad123a : killed         ,  55.079813 s
mozilla-try     x00000_revs_x00000_added_0_copies     dc8a3ca7010e d16fde900c9c :  19.647038 s,   1.442793 s, -18.204245 s, × 0.0734


This changeset compared to the Python Code
==========================================


Repo            Cases                                 Source-Rev   Dest-Rev         Py-time       Rust-time    Difference   Factor
------------------------------------------------------------------------------------------------------------------------------------
mercurial       x_revs_x_added_0_copies               ad6b123de1c7 39cfcef4f463 :   0.000044 s,   0.000049 s,  +0.000005 s, × 1.1136
mercurial       x_revs_x_added_x_copies               2b1c78674230 0c1d10351869 :   0.000138 s,   0.000179 s,  +0.000041 s, × 1.2971
mercurial       x000_revs_x000_added_x_copies         81f8ff2a9bf2 dd3267698d84 :   0.005052 s,   0.006494 s,  +0.001442 s, × 1.2854
pypy            x_revs_x_added_0_copies               aed021ee8ae8 099ed31b181b :   0.000219 s,   0.000339 s,  +0.000120 s, × 1.5479
pypy            x_revs_x000_added_0_copies            4aa4e1f8e19a 359343b9ac0e :   0.000055 s,   0.000057 s,  +0.000002 s, × 1.0364
pypy            x_revs_x_added_x_copies               ac52eb7bbbb0 72e022663155 :   0.000128 s,   0.000299 s,  +0.000171 s, × 2.3359
pypy            x_revs_x00_added_x_copies             c3b14617fbd7 ace7255d9a26 :   0.001089 s,   0.001200 s,  +0.000111 s, × 1.1019
pypy            x_revs_x000_added_x000_copies         df6f7a526b60 a83dc6a2d56f :   0.017407 s,   0.025120 s,  +0.007713 s, × 1.4431
pypy            x000_revs_xx00_added_0_copies         89a76aede314 2f22446ff07e :   0.094175 s,   0.506921 s,  +0.412746 s, × 5.3828
pypy            x000_revs_x000_added_x_copies         8a3b5bfd266e 2c68e87c3efe :   0.238009 s,   1.272060 s,  +1.034051 s, × 5.3446
pypy            x000_revs_x000_added_x000_copies      89a76aede314 7b3dda341c84 :   0.125876 s,   0.690941 s,  +0.565065 s, × 5.4891
pypy            x0000_revs_x_added_0_copies           d1defd0dc478 c9cb1334cc78 :   3.581556 s,  33.527067 s, +29.945511 s, × 9.3610
pypy            x0000_revs_xx000_added_0_copies       bf2c629d0071 4ffed77c095c :   0.016721 s,   0.021970 s,  +0.005249 s, × 1.3139
pypy            x0000_revs_xx000_added_x000_copies    08ea3258278e d9fa043f30c0 :   0.242367 s,   1.772094 s,  +1.529727 s, × 7.3116
netbeans        x_revs_x_added_0_copies               fb0955ffcbcd a01e9239f9e7 :   0.000165 s,   0.000185 s,  +0.000020 s, × 1.1212
netbeans        x_revs_x000_added_0_copies            6f360122949f 20eb231cc7d0 :   0.000114 s,   0.000135 s,  +0.000021 s, × 1.1842
netbeans        x_revs_x_added_x_copies               1ada3faf6fb6 5a39d12eecf4 :   0.000296 s,   0.000329 s,  +0.000033 s, × 1.1115
netbeans        x_revs_x00_added_x_copies             35be93ba1e2c 9eec5e90c05f :   0.001124 s,   0.001343 s,  +0.000219 s, × 1.1948
netbeans        x000_revs_xx00_added_0_copies         eac3045b4fdd 51d4ae7f1290 :   0.013060 s,   0.029396 s,  +0.016336 s, × 2.2508
netbeans        x000_revs_x000_added_x_copies         e2063d266acd 6081d72689dc :   0.017112 s,   0.040210 s,  +0.023098 s, × 2.3498
netbeans        x000_revs_x000_added_x000_copies      ff453e9fee32 411350406ec2 :   0.660350 s,   4.556794 s,  +3.896444 s, × 6.9006
netbeans        x0000_revs_xx000_added_x000_copies    588c2d1ced70 1aad62e59ddd :  10.032499 s, killed
mozilla-central x_revs_x_added_0_copies               3697f962bb7b 7015fcdd43a2 :   0.000189 s,   0.000199 s,  +0.000010 s, × 1.0529
mozilla-central x_revs_x000_added_0_copies            dd390860c6c9 40d0c5bed75d :   0.000462 s,   0.000639 s,  +0.000177 s, × 1.3831
mozilla-central x_revs_x_added_x_copies               8d198483ae3b 14207ffc2b2f :   0.000270 s,   0.000542 s,  +0.000272 s, × 2.0074
mozilla-central x_revs_x00_added_x_copies             98cbc58cc6bc 446a150332c3 :   0.001474 s,   0.001685 s,  +0.000211 s, × 1.1431
mozilla-central x_revs_x000_added_x000_copies         3c684b4b8f68 0a5e72d1b479 :   0.004806 s,   0.006954 s,  +0.002148 s, × 1.4469
mozilla-central x_revs_x0000_added_x0000_copies       effb563bb7e5 c07a39dc4e80 :   0.085150 s,   0.132938 s,  +0.047788 s, × 1.5612
mozilla-central x000_revs_xx00_added_0_copies         6100d773079a 04a55431795e :   0.007064 s,   0.008683 s,  +0.001619 s, × 1.2292
mozilla-central x000_revs_x000_added_x_copies         9f17a6fc04f9 2d37b966abed :   0.004741 s,   0.005956 s,  +0.001215 s, × 1.2563
mozilla-central x000_revs_x000_added_x000_copies      7c97034feb78 4407bd0c6330 :   0.190133 s,   0.963905 s,  +0.773772 s, × 5.0696
mozilla-central x0000_revs_xx000_added_0_copies       9eec5917337d 67118cc6dcad :   0.035651 s,   0.049239 s,  +0.013588 s, × 1.3811
mozilla-central x0000_revs_xx000_added_x000_copies    f78c615a656c 96a38b690156 :   0.440694 s,   4.217003 s,  +3.776309 s, × 9.5690
mozilla-central x00000_revs_x0000_added_x0000_copies  6832ae71433c 4c222a1d9a00 :  18.454163 s, killed
mozilla-central x00000_revs_x00000_added_x000_copies  76caed42cf7c 1daa622bbe42 :  31.562719 s, killed
mozilla-try     x_revs_x_added_0_copies               aaf6dde0deb8 9790f499805a :   0.001189 s,   0.001197 s,  +0.000008 s, × 1.0067
mozilla-try     x_revs_x000_added_0_copies            d8d0222927b4 5bb8ce8c7450 :   0.001204 s,   0.001213 s,  +0.000009 s, × 1.0075
mozilla-try     x_revs_x_added_x_copies               092fcca11bdb 936255a0384a :   0.000586 s,   0.000762 s,  +0.000176 s, × 1.3003
mozilla-try     x_revs_x00_added_x_copies             b53d2fadbdb5 017afae788ec :   0.001845 s,   0.001909 s,  +0.000064 s, × 1.0347
mozilla-try     x_revs_x000_added_x000_copies         20408ad61ce5 6f0ee96e21ad :   0.063822 s,   0.093021 s,  +0.029199 s, × 1.4575
mozilla-try     x_revs_x0000_added_x0000_copies       effb563bb7e5 c07a39dc4e80 :   0.088038 s,   0.134536 s,  +0.046498 s, × 1.5282
mozilla-try     x000_revs_xx00_added_0_copies         6100d773079a 04a55431795e :   0.007389 s,   0.009071 s,  +0.001682 s, × 1.2276
mozilla-try     x000_revs_x000_added_x_copies         9f17a6fc04f9 2d37b966abed :   0.004868 s,   0.006206 s,  +0.001338 s, × 1.2749
mozilla-try     x000_revs_x000_added_x000_copies      1346fd0130e4 4c65cbdabc1f :   0.222450 s,   1.150502 s,  +0.928052 s, × 5.1720
mozilla-try     x0000_revs_x_added_0_copies           63519bfd42ee a36a2a865d92 :   0.370675 s,   1.114864 s,  +0.744189 s, × 3.0077
mozilla-try     x0000_revs_x_added_x_copies           9fe69ff0762d bcabf2a78927 :   0.358020 s,   1.042658 s,  +0.684638 s, × 2.9123
mozilla-try     x0000_revs_xx000_added_x_copies       156f6e2674f2 4d0f2c178e66 :   0.145235 s,   0.447402 s,  +0.302167 s, × 3.0805
mozilla-try     x0000_revs_xx000_added_0_copies       9eec5917337d 67118cc6dcad :   0.037606 s,   0.051132 s,  +0.013526 s, × 1.3597
mozilla-try     x0000_revs_xx000_added_x000_copies    89294cd501d9 7ccb2fc7ccb5 :   7.382439 s,  83.508590 s, +76.126151 s, × 11.3118
mozilla-try     x0000_revs_x0000_added_x0000_copies   e928c65095ed e951f4ad123a :   7.273506 s,  55.079813 s, +47.806307 s, × 7.5727
mozilla-try     x00000_revs_x00000_added_0_copies     dc8a3ca7010e d16fde900c9c :   1.074593 s,   1.442793 s,  +0.368200 s, × 1.3426
mozilla-try     x00000_revs_x0000_added_x0000_copies  8d3fafa80d4b eb884023b810 :  27.746195 s, killed

Differential Revision: https://phab.mercurial-scm.org/D9300

use crate::utils::hg_path::HgPathBuf;
use crate::Revision;

use im_rc::ordmap::OrdMap;

use std::collections::HashMap;
use std::collections::HashSet;

pub type PathCopies = HashMap<HgPathBuf, HgPathBuf>;

#[derive(Clone, Debug)]
struct TimeStampedPathCopy {
    /// revision at which the copy information was added
    rev: Revision,
    /// the copy source, (Set to None in case of deletion of the associated
    /// key)
    path: Option<HgPathBuf>,
}

/// maps CopyDestination to Copy Source (+ a "timestamp" for the operation)
type TimeStampedPathCopies = OrdMap<HgPathBuf, TimeStampedPathCopy>;

/// hold parent 1, parent 2 and relevant files actions.
pub type RevInfo = (Revision, Revision, ChangedFiles);

/// represent the files affected by a changesets
///
/// This hold a subset of mercurial.metadata.ChangingFiles as we do not need
/// all the data categories tracked by it.
pub struct ChangedFiles {
    removed: HashSet<HgPathBuf>,
    merged: HashSet<HgPathBuf>,
    salvaged: HashSet<HgPathBuf>,
    copied_from_p1: PathCopies,
    copied_from_p2: PathCopies,
}

impl ChangedFiles {
    pub fn new(
        removed: HashSet<HgPathBuf>,
        merged: HashSet<HgPathBuf>,
        salvaged: HashSet<HgPathBuf>,
        copied_from_p1: PathCopies,
        copied_from_p2: PathCopies,
    ) -> Self {
        ChangedFiles {
            removed,
            merged,
            salvaged,
            copied_from_p1,
            copied_from_p2,
        }
    }

    pub fn new_empty() -> Self {
        ChangedFiles {
            removed: HashSet::new(),
            merged: HashSet::new(),
            salvaged: HashSet::new(),
            copied_from_p1: PathCopies::new(),
            copied_from_p2: PathCopies::new(),
        }
    }
}

/// Same as mercurial.copies._combine_changeset_copies, but in Rust.
///
/// Arguments are:
///
/// revs: all revisions to be considered
/// children: a {parent ? [childrens]} mapping
/// target_rev: the final revision we are combining copies to
/// rev_info(rev): callback to get revision information:
///   * first parent
///   * second parent
///   * ChangedFiles
/// isancestors(low_rev, high_rev): callback to check if a revision is an
///                                 ancestor of another
pub fn combine_changeset_copies(
    revs: Vec<Revision>,
    children: HashMap<Revision, Vec<Revision>>,
    target_rev: Revision,
    rev_info: &impl Fn(Revision) -> RevInfo,
    is_ancestor: &impl Fn(Revision, Revision) -> bool,
) -> PathCopies {
    let mut all_copies = HashMap::new();

    for rev in revs {
        // Retrieve data computed in a previous iteration
        let copies = all_copies.remove(&rev);
        let copies = match copies {
            Some(c) => c,
            None => TimeStampedPathCopies::default(), // root of the walked set
        };

        let current_children = match children.get(&rev) {
            Some(c) => c,
            None => panic!("inconsistent `revs` and `children`"),
        };

        for child in current_children {
            // We will chain the copies information accumulated for `rev` with
            // the individual copies information for each of its children.
            // Creating a new PathCopies for each `rev` ? `children` vertex.
            let (p1, p2, changes) = rev_info(*child);

            let (parent, child_copies) = if rev == p1 {
                (1, &changes.copied_from_p1)
            } else {
                assert_eq!(rev, p2);
                (2, &changes.copied_from_p2)
            };
            let mut new_copies = copies.clone();

            for (dest, source) in child_copies {
                let entry;
                if let Some(v) = copies.get(source) {
                    entry = match &v.path {
                        Some(path) => Some((*(path)).to_owned()),
                        None => Some(source.to_owned()),
                    }
                } else {
                    entry = Some(source.to_owned());
                }
                // Each new entry is introduced by the children, we record this
                // information as we will need it to take the right decision
                // when merging conflicting copy information. See
                // merge_copies_dict for details.
                let ttpc = TimeStampedPathCopy {
                    rev: *child,
                    path: entry,
                };
                new_copies.insert(dest.to_owned(), ttpc);
            }

            // We must drop copy information for removed file.
            //
            // We need to explicitly record them as dropped to propagate this
            // information when merging two TimeStampedPathCopies object.
            for f in changes.removed.iter() {
                if new_copies.contains_key(f.as_ref()) {
                    let ttpc = TimeStampedPathCopy {
                        rev: *child,
                        path: None,
                    };
                    new_copies.insert(f.to_owned(), ttpc);
                }
            }

            // Merge has two parents needs to combines their copy information.
            //
            // If the vertex from the other parent was already processed, we
            // will have a value for the child ready to be used. We need to
            // grab it and combine it with the one we already
            // computed. If not we can simply store the newly
            // computed data. The processing happening at
            // the time of the second parent will take care of combining the
            // two TimeStampedPathCopies instance.
            match all_copies.remove(child) {
                None => {
                    all_copies.insert(child, new_copies);
                }
                Some(other_copies) => {
                    let (minor, major) = match parent {
                        1 => (other_copies, new_copies),
                        2 => (new_copies, other_copies),
                        _ => unreachable!(),
                    };
                    let merged_copies =
                        merge_copies_dict(minor, major, &changes, is_ancestor);
                    all_copies.insert(child, merged_copies);
                }
            };
        }
    }

    // Drop internal information (like the timestamp) and return the final
    // mapping.
    let tt_result = all_copies
        .remove(&target_rev)
        .expect("target revision was not processed");
    let mut result = PathCopies::default();
    for (dest, tt_source) in tt_result {
        if let Some(path) = tt_source.path {
            result.insert(dest, path);
        }
    }
    result
}

/// merge two copies-mapping together, minor and major
///
/// In case of conflict, value from "major" will be picked, unless in some
/// cases. See inline documentation for details.
#[allow(clippy::if_same_then_else)]
fn merge_copies_dict(
    minor: TimeStampedPathCopies,
    major: TimeStampedPathCopies,
    changes: &ChangedFiles,
    is_ancestor: &impl Fn(Revision, Revision) -> bool,
) -> TimeStampedPathCopies {
    let mut result = minor.clone();
    for (dest, src_major) in major {
        let overwrite;
        if let Some(src_minor) = minor.get(&dest) {
            if src_major.path == src_minor.path {
                // we have the same value, but from other source;
                if src_major.rev == src_minor.rev {
                    // If the two entry are identical, no need to do anything
                    overwrite = false;
                } else if is_ancestor(src_major.rev, src_minor.rev) {
                    overwrite = false;
                } else {
                    overwrite = true;
                }
            } else if src_major.rev == src_minor.rev {
                // We cannot get copy information for both p1 and p2 in the
                // same rev. So this is the same value.
                overwrite = false;
            } else if src_major.path.is_none()
                && changes.salvaged.contains(&dest)
            {
                // If the file is "deleted" in the major side but was salvaged
                // by the merge, we keep the minor side alive
                overwrite = false;
            } else if src_minor.path.is_none()
                && changes.salvaged.contains(&dest)
            {
                // If the file is "deleted" in the minor side but was salvaged
                // by the merge, unconditionnaly preserve the major side.
                overwrite = true;
            } else if changes.merged.contains(&dest) {
                // If the file was actively merged, copy information from each
                // side might conflict. The major side will win such conflict.
                overwrite = true;
            } else if is_ancestor(src_major.rev, src_minor.rev) {
                // If the minor side is strictly newer than the major side, it
                // should be kept.
                overwrite = false;
            } else if src_major.path.is_some() {
                // without any special case, the "major" value win other the
                // "minor" one.
                overwrite = true;
            } else if is_ancestor(src_minor.rev, src_major.rev) {
                // the "major" rev is a direct ancestors of "minor", any
                // different value should overwrite
                overwrite = true;
            } else {
                // major version is None (so the file was deleted on that
                // branch) annd that branch is independant (neither minor nor
                // major is an ancestors of the other one.) We preserve the new
                // information about the new file.
                overwrite = false;
            }
        } else {
            // minor had no value
            overwrite = true;
        }
        if overwrite {
            result.insert(dest, src_major);
        }
    }
    result
}