Add Convex Hull in C (#3958)

---------

Co-authored-by: rzuckerm <[email protected]>

archive/c/c/convex-hull.c

@@ -0,0 +1,184 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdbool.h>
+#include <ctype.h>
+
+typedef struct {
+    int x;
+    int y;
+} Point;
+
+void printUsageAndExit() {
+    printf("Usage: please provide at least 3 x and y coordinates as separate lists (e.g. \"100, 440, 210\")\n");
+    exit(1);
+}
+
+int compare(const void *p1, const void *p2) {
+    Point *point1 = (Point *)p1;
+    Point *point2 = (Point *)p2;
+
+    if (point1->x != point2->x) {
+        return point1->x - point2->x;
+    }
+    return point1->y - point2->y;
+}
+
+int orientation(Point p, Point q, Point r) {
+    int val = (q.y - p.y) * (r.x - q.x) - (q.x - p.x) * (r.y - q.y);
+    return (val == 0) ? 0 : (val > 0) ? 1 : 2;
+}
+
+void convexHull(Point points[], int n) {
+    Point hull[n];
+    int hullCount = 0;
+
+    qsort(points, n, sizeof(Point), compare);
+
+    int l = 0;
+    for (int i = 1; i < n; i++)
+        if (points[i].x < points[l].x)
+            l = i;
+
+    int p = l, q;
+    do {
+        hull[hullCount++] = points[p];
+        q = (p + 1) % n;
+
+        for (int i = 0; i < n; i++) {
+            if (orientation(points[p], points[i], points[q]) == 2) {
+                q = i;
+            }
+        }
+
+        p = q;
+    } while (p != l);
+
+    for (int i = 0; i < hullCount; i++) {
+        printf("(%d, %d)\n", hull[i].x, hull[i].y);
+    }
+}
+
+// Function to check if a string is a valid number
+bool isInteger(const char *s) {
+    char *end;
+    strtol(s, &end, 10); // Convert string to long
+    return (*end == '\0' || *end == '\n'); // Check if the entire string was valid
+}
+
+// Function to trim whitespace from a string
+char* trimWhitespace(char *str) {
+    // Trim leading whitespace
+    while (isspace((unsigned char)*str)) str++;
+    // Trim trailing whitespace
+    char *end = str + strlen(str) - 1;
+    while (end > str && isspace((unsigned char)*end)) end--;
+    *(end + 1) = '\0'; // Null terminate after the last non-space character
+    return str;
+}
+
+// Function to parse input string and populate the array
+int parseInput(const char *input, int **arr, int *size) {
+    char *token;
+    int capacity = 10; // Initial capacity
+    *arr = malloc(capacity * sizeof(int));
+    if (*arr == NULL) {
+        return false;
+    }
+
+    // Tokenize the input string based on commas
+    char *inputCopy = strdup(input);
+    if (inputCopy == NULL) {
+        free(*arr);
+        *arr = NULL;
+        return false;
+    }
+
+    token = strtok(inputCopy, ",");
+    *size = 0;
+    while (token) {
+        trimWhitespace(token); // Trim whitespace around token
+        if (!isInteger(token)) {
+            free(*arr);
+            free(inputCopy);
+            *arr = NULL;
+            return false; // Exit if a number is invalid
+        }
+
+        if (*size >= capacity) {
+            capacity *= 2;
+            *arr = realloc(*arr, capacity * sizeof(int));
+            if (*arr == NULL) {
+                free(inputCopy);
+                return false;
+            }
+        }
+        (*arr)[(*size)++] = atoi(token);
+        token = strtok(NULL, ",");
+    }
+
+    // Resize the array to the actual size
+    *arr = realloc(*arr, *size * sizeof(int));
+    free(inputCopy); // Free the input copy
+    if (*arr == NULL) {
+        return false;
+    }
+
+    return true; // Successful parsing
+}
+
+void parseCoordinates(char *inputX, char *inputY) {
+    int *xCoords = NULL;
+    int *yCoords = NULL;
+    int xSize = 0;
+    int ySize = 0;
+    if (!parseInput(inputX, &xCoords, &xSize) ||
+        !parseInput(inputY, &yCoords, &ySize) ||
+        xSize != ySize ||
+        xSize < 3) {
+        if (xCoords != NULL) {
+            free(xCoords);
+        }
+
+        if (yCoords != NULL) {
+            free(yCoords);
+        }
+
+        printUsageAndExit();
+    }
+
+    int count = xSize;
+    Point *points = malloc(sizeof(Point) * count);
+    if (points == NULL) {
+        free(xCoords);
+        free(yCoords);
+        printUsageAndExit();
+    }
+
+    for (int i = 0; i < count; i++) {
+        points[i].x = xCoords[i];
+        points[i].y = yCoords[i];
+    }
+
+    free(xCoords);
+    free(yCoords);
+
+    convexHull(points, count);
+    free(points);
+}
+
+int main(int argc, char *argv[]) {
+    if (argc != 3) {
+        printUsageAndExit();
+    }
+
+    char *inputX = argv[1];
+    char *inputY = argv[2];
+
+    if (strlen(inputX) == 0 || strlen(inputY) == 0) {
+        printUsageAndExit();
+    }
+
+    parseCoordinates(inputX, inputY);
+    return 0;
+}