feat: Add simple brainfuck interpreter for testing

brainfuck/main.c

@@ -1,40 +1,147 @@
 #include <stdio.h>
 #include <string.h>
+#include <stdlib.h>
 
 #define BAND_LENGTH (30000)
 
 char band[BAND_LENGTH];
-size_t position = 0;
+long long position = 0; // signed
 
 void init_band() {
     memset(band, 0, BAND_LENGTH);
 }
 
-int error(const char* msg) {
+int max(int a, int b) {
+    return a > b ? a : b;
+}
+
+int error(const char* msg, const char* input, size_t offset) {
     fprintf(stderr, "%s\n", msg);
+    if (input) {
+        const char* display_segment = input;
+        int display_offset = offset;
+        if (offset > 5) {
+            display_segment += offset - 5;
+            display_offset = 5;
+        }
+        fprintf(stderr, "input position: %zu\n", offset);
+        fprintf(stderr, "         %.*s%c\n", display_offset, "           ", input[offset]);
+        fprintf(stderr, "context: %.10s\n", display_segment);
+        fprintf(stderr, "         %.*s^\n", display_offset,  "          ");
+        fprintf(stderr, "position: %lld\n", position);
+    }
     return 1;
 }
 
-int run(FILE* input) {
-    int c;
-    while((c = getc(input)) != EOF) {
-        switch (c) {
+size_t search_loop_end(const char* input, size_t length, size_t start) {
+    size_t nested_loops = 0;
+    for (size_t i = start + 1; i < length; i++) {
+        if (input[i] == '[') {
+            nested_loops++;
+        } else if (input[i] == ']') {
+            if (nested_loops == 0) {
+                return i;
+            } else {
+                nested_loops--;
+            }
+        }
+    }
+    return start;
+}
+
+size_t search_loop_begin(const char* input, size_t start) {
+    size_t nested_loops = 0;
+    for (long long i = start - 1; i >= 0; i--) {
+        if (input[i] == ']') {
+            nested_loops++;
+        } else if (input[i] == '[') {
+            if (nested_loops == 0) {
+                return i;
+            } else {
+                nested_loops--;
+            }
+        }
+    }
+    return start;
+}
+
+int run(const char* input) {
+    size_t program_counter = 0;
+    size_t input_length = strlen(input);
+
+    while(program_counter < input_length) {
+        switch (input[program_counter]) {
             case '+': band[position]++; break;
             case '-': band[position]--; break;
             case '<':
-                if (--position < 0) return error("band underflow");
+                if (--position < 0) return error("band underflow", input, program_counter);
                 break;
             case '>':
-                if (++position >= BAND_LENGTH) return error("band overflow");
+                if (++position >= BAND_LENGTH) return error("band overflow", input, program_counter);
                 break;
             case '.': putchar(band[position]); break;
             case ',': band[position] = getchar(); break;
+            case '[':
+                if (!band[position]) {
+                    size_t target = search_loop_end(input, input_length, program_counter);
+                    if (target == program_counter) {
+                        return error("no matching ] found", input, program_counter);
+                    }
+                    program_counter = target;
+                }
+                break;
+            case ']':
+                if (band[position]) {
+                    size_t target = search_loop_begin(input, program_counter);
+                    if (target == program_counter) {
+                        return error("no matching [ found", input, program_counter);
+                    }
+                    program_counter = target;
+                }
+                break;
         }
+
+        program_counter++;
     }
 
     return 0;
 }
 
+char* read_to_string(FILE* input) {
+    size_t buffer_used = 0;
+    size_t buffer_size = 1024;
+    char* buffer = malloc(buffer_size);
+    if (!buffer) {
+        return NULL;
+    }
+
+    int c;
+    while ((c = getc(input)) != EOF) {
+        switch(c) {
+            case '+':
+            case '-':
+            case '>':
+            case '<':
+            case '[':
+            case ']':
+            case '.':
+            case ',':
+                buffer[buffer_used++] = c;
+                if (buffer_used >= buffer_size) {
+                    buffer_size *= 2;
+                    buffer = realloc(buffer, buffer_size);
+                    if (!buffer) {
+                        return NULL;
+                    }
+                }
+                break;
+        }
+    }
+
+    buffer[buffer_used] = '\0';
+    return buffer;
+}
+
 int main(int argc, char** argv) {
     if (argc != 2) {
         return 1;
@@ -47,5 +154,15 @@ int main(int argc, char** argv) {
     }
 
     init_band();
-    return run(input);
+
+    char* program = read_to_string(input);
+    if (!program) {
+        return error("error reading input", NULL, 0);
+    }
+
+    int result = run(program);
+
+    free(program);
+
+    return result;
 }