Merge pull request #243520 from tweag/lib.lists.commonPrefix

`lib.lists.{findFirstIndex,commonPrefix}`: init
This commit is contained in:
Silvan Mosberger 2023-07-26 23:14:58 +02:00 committed by GitHub
commit bd74cdfb2e
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2 changed files with 124 additions and 29 deletions

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@ -3,7 +3,7 @@
{ lib }:
let
inherit (lib.strings) toInt;
inherit (lib.trivial) compare min;
inherit (lib.trivial) compare min id;
inherit (lib.attrsets) mapAttrs;
in
rec {
@ -180,18 +180,18 @@ rec {
else if len != 1 then multiple
else head found;
/* Find the first element in the list matching the specified
/* Find the first index in the list matching the specified
predicate or return `default` if no such element exists.
Type: findFirst :: (a -> bool) -> a -> [a] -> a
Type: findFirstIndex :: (a -> Bool) -> b -> [a] -> (Int | b)
Example:
findFirst (x: x > 3) 7 [ 1 6 4 ]
=> 6
findFirst (x: x > 9) 7 [ 1 6 4 ]
=> 7
findFirstIndex (x: x > 3) null [ 0 6 4 ]
=> 1
findFirstIndex (x: x > 9) null [ 0 6 4 ]
=> null
*/
findFirst =
findFirstIndex =
# Predicate
pred:
# Default value to return
@ -229,7 +229,33 @@ rec {
if resultIndex < 0 then
default
else
elemAt list resultIndex;
resultIndex;
/* Find the first element in the list matching the specified
predicate or return `default` if no such element exists.
Type: findFirst :: (a -> bool) -> a -> [a] -> a
Example:
findFirst (x: x > 3) 7 [ 1 6 4 ]
=> 6
findFirst (x: x > 9) 7 [ 1 6 4 ]
=> 7
*/
findFirst =
# Predicate
pred:
# Default value to return
default:
# Input list
list:
let
index = findFirstIndex pred null list;
in
if index == null then
default
else
elemAt list index;
/* Return true if function `pred` returns true for at least one
element of `list`.
@ -637,6 +663,32 @@ rec {
else if start + count > len then len - start
else count);
/* The common prefix of two lists.
Type: commonPrefix :: [a] -> [a] -> [a]
Example:
commonPrefix [ 1 2 3 4 5 6 ] [ 1 2 4 8 ]
=> [ 1 2 ]
commonPrefix [ 1 2 3 ] [ 1 2 3 4 5 ]
=> [ 1 2 3 ]
commonPrefix [ 1 2 3 ] [ 4 5 6 ]
=> [ ]
*/
commonPrefix =
list1:
list2:
let
# Zip the lists together into a list of booleans whether each element matches
matchings = zipListsWith (fst: snd: fst != snd) list1 list2;
# Find the first index where the elements don't match,
# which will then also be the length of the common prefix.
# If all elements match, we fall back to the length of the zipped list,
# which is the same as the length of the smaller list.
commonPrefixLength = findFirstIndex id (length matchings) matchings;
in
take commonPrefixLength list1;
/* Return the last element of a list.
This function throws an error if the list is empty.

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@ -500,6 +500,39 @@ runTests {
expected = { a = [ 2 3 ]; b = [7]; c = [8];};
};
testListCommonPrefixExample1 = {
expr = lists.commonPrefix [ 1 2 3 4 5 6 ] [ 1 2 4 8 ];
expected = [ 1 2 ];
};
testListCommonPrefixExample2 = {
expr = lists.commonPrefix [ 1 2 3 ] [ 1 2 3 4 5 ];
expected = [ 1 2 3 ];
};
testListCommonPrefixExample3 = {
expr = lists.commonPrefix [ 1 2 3 ] [ 4 5 6 ];
expected = [ ];
};
testListCommonPrefixEmpty = {
expr = lists.commonPrefix [ ] [ 1 2 3 ];
expected = [ ];
};
testListCommonPrefixSame = {
expr = lists.commonPrefix [ 1 2 3 ] [ 1 2 3 ];
expected = [ 1 2 3 ];
};
testListCommonPrefixLazy = {
expr = lists.commonPrefix [ 1 ] [ 1 (abort "lib.lists.commonPrefix shouldn't evaluate this")];
expected = [ 1 ];
};
# This would stack overflow if `commonPrefix` were implemented using recursion
testListCommonPrefixLong =
let
longList = genList (n: n) 100000;
in {
expr = lists.commonPrefix longList longList;
expected = longList;
};
testSort = {
expr = sort builtins.lessThan [ 40 2 30 42 ];
expected = [2 30 40 42];
@ -530,45 +563,55 @@ runTests {
expected = false;
};
testFindFirstExample1 = {
expr = findFirst (x: x > 3) 7 [ 1 6 4 ];
expected = 6;
testFindFirstIndexExample1 = {
expr = lists.findFirstIndex (x: x > 3) (abort "index found, so a default must not be evaluated") [ 1 6 4 ];
expected = 1;
};
testFindFirstExample2 = {
expr = findFirst (x: x > 9) 7 [ 1 6 4 ];
expected = 7;
testFindFirstIndexExample2 = {
expr = lists.findFirstIndex (x: x > 9) "a very specific default" [ 1 6 4 ];
expected = "a very specific default";
};
testFindFirstEmpty = {
expr = findFirst (abort "when the list is empty, the predicate is not needed") null [];
testFindFirstIndexEmpty = {
expr = lists.findFirstIndex (abort "when the list is empty, the predicate is not needed") null [];
expected = null;
};
testFindFirstSingleMatch = {
expr = findFirst (x: x == 5) null [ 5 ];
expected = 5;
testFindFirstIndexSingleMatch = {
expr = lists.findFirstIndex (x: x == 5) null [ 5 ];
expected = 0;
};
testFindFirstSingleDefault = {
expr = findFirst (x: false) null [ (abort "if the predicate doesn't access the value, it must not be evaluated") ];
testFindFirstIndexSingleDefault = {
expr = lists.findFirstIndex (x: false) null [ (abort "if the predicate doesn't access the value, it must not be evaluated") ];
expected = null;
};
testFindFirstNone = {
expr = builtins.tryEval (findFirst (x: x == 2) null [ 1 (throw "the last element must be evaluated when there's no match") ]);
testFindFirstIndexNone = {
expr = builtins.tryEval (lists.findFirstIndex (x: x == 2) null [ 1 (throw "the last element must be evaluated when there's no match") ]);
expected = { success = false; value = false; };
};
# Makes sure that the implementation doesn't cause a stack overflow
testFindFirstBig = {
expr = findFirst (x: x == 1000000) null (range 0 1000000);
testFindFirstIndexBig = {
expr = lists.findFirstIndex (x: x == 1000000) null (range 0 1000000);
expected = 1000000;
};
testFindFirstLazy = {
expr = findFirst (x: x == 1) 7 [ 1 (abort "list elements after the match must not be evaluated") ];
expected = 1;
testFindFirstIndexLazy = {
expr = lists.findFirstIndex (x: x == 1) null [ 1 (abort "list elements after the match must not be evaluated") ];
expected = 0;
};
testFindFirstExample1 = {
expr = lists.findFirst (x: x > 3) 7 [ 1 6 4 ];
expected = 6;
};
testFindFirstExample2 = {
expr = lists.findFirst (x: x > 9) 7 [ 1 6 4 ];
expected = 7;
};
# ATTRSETS