Add generic utility function to find a value's position in an array

src/utilities.h

+/*******************************************************************************
+ * This file is part of SWIFT.
+ * Copyright (c) 2018 Jacob Kegerreis (jacob.kegerreis@durham.ac.uk)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ ******************************************************************************/
+
+#ifndef SWIFT_UTILITIES_H
+#define SWIFT_UTILITIES_H
+
+/**
+ * @brief Search for a value in a monotonic array to find the index such that
+ *      array[index] < value < array[index + 1]
+ *
+ * @param x The value to find
+ * @param arr The array to search
+ * @param n The length of the array
+ * @param i The found index
+ *
+ * Set -1 and n for x below and above the array edge values respectively.
+ */
+void find_value_in_monotonic_array(float x, float *arr, int n, int *i) {
+
+    int is_incr = (arr[n-1] > arr[0]);  // Increasing or decreasing?
+    int i_mid, i_low = 0, i_high = n;
+
+    // Until arr[i_low] < x < arr[i_high=i_low+1]
+    while (i_high - i_low > 1) {
+        i_mid = (i_high + i_low) >> 1;  // Middle index
+
+        // If mid < x and increasing or x < mid and decreasing
+        if ((arr[i_mid] <= x) == is_incr)
+            i_low = i_mid;
+        else
+            i_high = i_mid;
+    }
+
+    // Set i with the found i_low, or an error value if outside the array
+    if (x < arr[0])
+        *i = -1;
+    else if (arr[n-1] <= x)
+        *i = n;
+    else
+        *i = i_low;
+}
+
+#endif /* SWIFT_UTILITIES_H */
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