diff --git a/src/calibration.c b/src/calibration.c
new file mode 100644
index 0000000..81d5117
--- /dev/null
+++ b/src/calibration.c
@@ -0,0 +1,80 @@
+#include <ctype.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+static char *strrev(char *to_reverse) {
+    size_t length = strlen(to_reverse);
+    int position = length - 1;
+    char *reversed = malloc((1 + length) * sizeof(char));
+
+    while (position >= 0) {
+        *reversed = to_reverse[position];
+        reversed++;
+        position--;
+    }
+    *reversed = '\0';
+    reversed = reversed - length;
+
+    return reversed;
+}
+
+static char first_digit(char *line) {
+    while (*line != '\0' && !isdigit(*line)) {
+        line++;
+    }
+
+    if (*line != '\0') {
+        return *line;
+    } else {
+        return 0;
+    }
+}
+
+static char *read_line(FILE *input_file) {
+    unsigned int max_length = 16;
+    unsigned int current_length = 0;
+    char *line = malloc(max_length * sizeof(char));
+    int current_char = fgetc(input_file);
+
+    while (current_char != EOF && current_char != '\n') {
+        if (current_length == max_length) {
+            max_length *= 2;
+            line = realloc(line, max_length * sizeof(char));
+        }
+
+        line[current_length] = current_char;
+        current_length++;
+        current_char = fgetc(input_file);
+    }
+
+    if (current_length == max_length) {
+        line = realloc(line, (max_length + 1) * sizeof(char));
+    }
+    line[current_length] = '\0';
+
+    return line;
+}
+
+unsigned int get_calibration_sum(FILE *input_file) {
+    unsigned int sum = 0;
+    char *current_line, *reversed_line;
+    char intermediate_buffer[3];
+    intermediate_buffer[2] = '\0';
+
+    while (!feof(input_file)) {
+        current_line = read_line(input_file);
+        reversed_line = strrev(current_line);
+        if (*current_line != '\0') {
+            intermediate_buffer[0] = first_digit(current_line);
+            intermediate_buffer[1] = first_digit(reversed_line);
+            sum += atoi(intermediate_buffer);
+        }
+        free(reversed_line);
+        free(current_line);
+
+        printf("sum: %d\n", sum);
+    }
+
+    return sum;
+}