From cc0d818b9782ed4cb85ceb18df3c4f8e5ab49b67 Mon Sep 17 00:00:00 2001
From: Mark Rogoyski <markrogoyski@gmail.com>
Date: Sat, 2 Mar 2024 15:44:15 -0800
Subject: [PATCH] Add Chebyshev Distance.

---
 src/Statistics/Distance.php       | 31 ++++++++++
 tests/Statistics/DistanceTest.php | 94 +++++++++++++++++++++++++++++++
 2 files changed, 125 insertions(+)

diff --git a/src/Statistics/Distance.php b/src/Statistics/Distance.php
index 8b569a6d7..553163da8 100644
--- a/src/Statistics/Distance.php
+++ b/src/Statistics/Distance.php
@@ -470,4 +470,35 @@ function (float $｜pᵢ − qᵢ｜, float $｜pᵢ｜ ＋ ｜qᵢ｜) {
             $｜p｜ ＋ ｜q｜
         ));
     }
+
+    /**
+     * Chebyshev Distance
+     * A metric defined on a real coordinate space where the distance between two points
+     * is the greatest of their differences along any coordinate dimension.
+     *
+     * D(x, y) = max(｜xᵢ − yᵢ｜)
+     *
+     * https://en.wikipedia.org/wiki/Chebyshev_distance
+     *
+     * @param array<float> $xs
+     * @param array<float> $ys
+     *
+     * @return float
+     *
+     * @throws Exception\BadDataException
+     */
+    public static function chebyshev(array $xs, array $ys): float
+    {
+        if (\count($xs) !== \count($ys)) {
+            throw new Exception\BadDataException('xs and ys must have the same number of elements');
+        }
+
+        return \max(\array_map(
+            function (float $xᵢ, $yᵢ) {
+                return \abs($xᵢ - $yᵢ);
+            },
+            $xs,
+            $ys
+        ));
+    }
 }
diff --git a/tests/Statistics/DistanceTest.php b/tests/Statistics/DistanceTest.php
index 6de04b048..264b76713 100644
--- a/tests/Statistics/DistanceTest.php
+++ b/tests/Statistics/DistanceTest.php
@@ -1252,4 +1252,98 @@ public function testCanberraExceptionDifferentNumberElements()
         // When
         $distance = Distance::canberra($p, $q);
     }
+
+    /**
+     * @test chebyshev
+     * @dataProvider dataProviderForChebyshev
+     * @param        array $x
+     * @param        array $y
+     * @param        float $expected
+     */
+    public function testChebyshev(array $x, array $y, float $expected): void
+    {
+        // When
+        $distance = Distance::chebyshev($x, $y);
+
+        // Then
+        $this->assertEqualsWithDelta($expected, $distance, 0.0001);
+    }
+
+    public function dataProviderForChebyshev(): array
+    {
+        return [
+            [
+                [0],
+                [0],
+                0
+            ],
+            [
+                [1],
+                [1],
+                0
+            ],
+            [
+                [1],
+                [0],
+                1
+            ],
+            [
+                [0],
+                [1],
+                1
+            ],
+            [
+                [1, 2],
+                [2, 4],
+                2
+            ],
+            [
+                [1, 2, 3],
+                [2, 4, 6],
+                3
+            ],
+            [
+                [0, 3, 4, 5],
+                [7, 6, 3, -1],
+                7
+            ],
+            [
+                [1, 2, 3, 4],
+                [-5, -6, 7, 8],
+                8
+            ],
+            [
+                [1, 5, 2, 3, 10],
+                [4, 15, 20, 5, 5],
+                18
+            ],
+            [
+                [1, 5, 2, 3, 10],
+                [1, 5, 2, 3, 10],
+                0
+            ],
+            [
+                [4, 15, 20, 5, 5],
+                [4, 15, 20, 5, 5],
+                0
+            ],
+        ];
+    }
+
+    /**
+     * @test   chebyshev exception when inputs are different lengths
+     * @throws Exception\BadDataException
+     */
+    public function testChebyshevExceptionDifferentNumberElements()
+    {
+        // Given
+        $xs = [1, 2, 3];
+        $ys = [2, 3];
+
+        // Then
+        $this->expectException(Exception\BadDataException::class);
+
+        // When
+        Distance::chebyshev($xs, $ys);
+    }
 }