1 files changed, 38 insertions, 0 deletions
diff --git a/src/approx_derivative.c b/src/approx_derivative.c
new file mode 100644
index 0000000..b33a208
--- /dev/null
+++ b/src/approx_derivative.c
@@ -0,0 +1,38 @@
+#include "lizfcm.h"
+#include <assert.h>
+
+double central_derivative_at(double (*f)(double), double a, double h) {
+  assert(h > 0);
+
+  double x2 = a + h;
+  double x1 = a - h;
+
+  double y2 = (*f)(x2);
+  double y1 = (*f)(x1);
+
+  return (y2 - y1) / (x2 - x1);
+}
+
+double forward_derivative_at(double (*f)(double), double a, double h) {
+  assert(h > 0);
+
+  double x2 = a + h;
+  double x1 = a;
+
+  double y2 = (*f)(x2);
+  double y1 = (*f)(x1);
+
+  return (y2 - y1) / (x2 - x1);
+}
+
+double backward_derivative_at(double (*f)(double), double a, double h) {
+  assert(h > 0);
+
+  double x2 = a;
+  double x1 = a - h;
+
+  double y2 = (*f)(x2);
+  double y1 = (*f)(x1);
+
+  return (y2 - y1) / (x2 - x1);
+}