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src/util/range_minimum_query.h

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+/*
+ * Copyright 2025 Valve Corporation
+ * SPDX-License-Identifier: MIT
+ */
+
+#ifndef RANGE_MINIMUM_QUERY_H
+#define RANGE_MINIMUM_QUERY_H
+
+#include <stdint.h>
+#include <string.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Find the smallest integer in a portion of an array.
+ *
+ * We use the simple RMQ algorithm that uses O(n log n) preprcessing time and
+ * O(1) query time (see eg. Bender and Colton section 3).
+ *
+ * Bender, M.A., Farach-Colton, M. (2000). The LCA Problem Revisited. In:
+ *     Gonnet, G.H., Viola, A. (eds) LATIN 2000: Theoretical Informatics. LATIN
+ *     2000. Lecture Notes in Computer Science, vol 1776. Springer, Berlin,
+ *     Heidelberg. https://doi.org/10.1007/10719839_9
+ */
+
+struct range_minimum_query_table {
+   uint32_t *table;
+   uint32_t width, height;
+};
+
+static inline void
+range_minimum_query_table_init(struct range_minimum_query_table *table)
+{
+   memset((void*) table, 0, sizeof(*table));
+}
+
+void
+range_minimum_query_table_resize(struct range_minimum_query_table *table,
+                                 void *mem_ctx, uint32_t width);
+
+/**
+ * Perform preprocessing on the table to ready it for queries.
+ *
+ * Takes O(n log n) time.
+ */
+void
+range_minimum_query_table_preprocess(struct range_minimum_query_table *table);
+
+/**
+ * Find the smallest value in the array among indices in the half-open interval
+ * [left_idx, right_idx).
+ *
+ * Takes O(1) time.
+ */
+uint32_t
+range_minimum_query(struct range_minimum_query_table *const table,
+                    uint32_t left_idx, uint32_t right_idx);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif