From c2a5894f2dbe9982830066ab9347b059e6e7d845 Mon Sep 17 00:00:00 2001
From: John Firebaugh <john.firebaugh@gmail.com>
Date: Tue, 25 Apr 2017 18:20:26 -0700
Subject: [core] Immutable Impls

---
 src/mbgl/util/longest_common_subsequence.hpp | 106 +++++++++++++++++++++++++++
 1 file changed, 106 insertions(+)
 create mode 100644 src/mbgl/util/longest_common_subsequence.hpp

(limited to 'src/mbgl/util/longest_common_subsequence.hpp')

diff --git a/src/mbgl/util/longest_common_subsequence.hpp b/src/mbgl/util/longest_common_subsequence.hpp
new file mode 100644
index 0000000000..ac127c6356
--- /dev/null
+++ b/src/mbgl/util/longest_common_subsequence.hpp
@@ -0,0 +1,106 @@
+#pragma once
+
+#include <cstddef>
+#include <functional>
+#include <iterator>
+#include <vector>
+
+namespace mbgl {
+
+/*
+    Computes the longest common subsequence (LCS) of sequences A and B, represented
+    by pairs of random access iterators. The result is output to the provided output
+    iterator. Equality of elements is determined by the provided comparator, defaulting
+    to ==.
+
+    The algorithm used is the O(ND) time and space algorithm from:
+
+        Myers, Eugene W. An O(ND) Difference Algorithm and Its Variations. Algorithmica
+        (1986) 1: 251. http://xmailserver.org/diff2.pdf
+
+    For understanding this algorithm, http://simplygenius.net/Article/DiffTutorial1 is
+    also helpful.
+
+    TODO: implement the O(N) space refinement presented in the paper.
+*/
+template <class InIt, class OutIt, class Equal>
+OutIt longest_common_subsequence(InIt a, InIt endA,
+                                 InIt b, InIt endB,
+                                 OutIt outIt,
+                                 Equal eq) {
+    const std::ptrdiff_t N = endA - a;
+    const std::ptrdiff_t M = endB - b;
+    const std::ptrdiff_t D = N + M;
+
+    if (D == 0) {
+        return outIt;
+    }
+
+    std::vector<std::vector<std::ptrdiff_t>> vs;
+
+    // Self-executing lambda to allow `return` to break from inner loop, and avoid shadowing `v`.
+    [&] () {
+        std::vector<std::ptrdiff_t> v;
+        v.resize(2 * D + 1);
+        v[1] = 0;
+
+        // Core of the algorithm: greedily find farthest-reaching D-paths for increasing
+        // values of D. Store the farthest-reaching endpoints found in each iteration for
+        // later reconstructing the LCS.
+        for (std::ptrdiff_t d = 0; d <= D; ++d) {
+            for (std::ptrdiff_t k = -d; k <= d; k += 2) {
+                std::ptrdiff_t x = (k == -d || (k != d && v.at(k - 1 + D) < v.at(k + 1 + D)))
+                    ? v.at(k + 1 + D)       // moving down
+                    : v.at(k - 1 + D) + 1;  // moving right
+
+                std::ptrdiff_t y = x - k;
+
+                while (x < N && y < M && eq(a[x], b[y])) {
+                    x++;
+                    y++;
+                }
+
+                v[k + D] = x;
+
+                if (x >= N && y >= M) {
+                    vs.push_back(v);
+                    return;
+                }
+            }
+
+            vs.push_back(v);
+        }
+    }();
+
+    std::ptrdiff_t x = N;
+    std::ptrdiff_t y = M;
+
+    using E = typename std::iterator_traits<InIt>::value_type;
+    std::vector<E> lcsReverse;
+
+    // Reconstruct the LCS using the farthest-reaching endpoints stored above.
+    for (std::ptrdiff_t d = vs.size() - 1; x > 0 || y > 0; --d) {
+        const std::vector<std::ptrdiff_t>& v = vs.at(d);
+        const std::ptrdiff_t k = x - y;
+        const bool down = (k == -d || (k != d && v.at(k - 1 + D) < v.at(k + 1 + D)));
+        const std::ptrdiff_t kPrev = down ? k + 1 : k - 1;
+
+        x = v.at(kPrev + D);
+        y = x - kPrev;
+
+        for (std::ptrdiff_t c = v[k + D]; c != (down ? x : x + 1); --c) {
+            lcsReverse.push_back(a[c - 1]);
+        }
+    }
+
+    return std::copy(lcsReverse.rbegin(), lcsReverse.rend(), outIt);
+}
+
+template < typename InIt, typename OutIt >
+OutIt longest_common_subsequence(InIt a, InIt endA,
+                                 InIt b, InIt endB,
+                                 OutIt outIt) {
+    return longest_common_subsequence(a, endA, b, endB, outIt, std::equal_to<>());
+}
+
+}
-- 
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