1 files changed, 70 insertions, 0 deletions
diff --git a/src/deriv.cpp b/src/deriv.cpp
new file mode 100644
index 00000000..0e285265
--- /dev/null
+++ b/src/deriv.cpp
@@ -0,0 +1,70 @@
+/* dervative of (*P->fwd) projection */
+#define PJ_LIB__
+
+#include <math.h>
+
+#include "projects.h"
+
+int pj_deriv(LP lp, double h, const PJ *P, struct DERIVS *der) {
+    XY t;
+    /* get rid of constness until we can do it for real */
+    PJ *Q = (PJ *) P;
+    if (nullptr==Q->fwd)
+        return 1;
+
+    lp.lam += h;
+    lp.phi += h;
+    if (fabs(lp.phi) > M_HALFPI)
+        return 1;
+
+    h += h;
+    t = (*Q->fwd)(lp, Q);
+    if (t.x == HUGE_VAL)
+        return 1;
+
+    der->x_l = t.x;
+    der->y_p = t.y;
+    der->x_p = t.x;
+    der->y_l = t.y;
+
+    lp.phi -= h;
+    if (fabs(lp.phi) > M_HALFPI)
+        return 1;
+
+    t = (*Q->fwd)(lp, Q);
+    if (t.x == HUGE_VAL)
+        return 1;
+
+    der->x_l += t.x;
+    der->y_p -= t.y;
+    der->x_p -= t.x;
+    der->y_l += t.y;
+
+    lp.lam -= h;
+    t = (*Q->fwd)(lp, Q);
+    if (t.x == HUGE_VAL)
+        return 1;
+
+    der->x_l -= t.x;
+    der->y_p -= t.y;
+    der->x_p -= t.x;
+    der->y_l -= t.y;
+
+    lp.phi += h;
+    t = (*Q->fwd)(lp, Q);
+    if (t.x == HUGE_VAL)
+        return 1;
+
+    der->x_l -= t.x;
+    der->y_p += t.y;
+    der->x_p += t.x;
+    der->y_l -= t.y;
+
+    h += h;
+    der->x_l /= h;
+    der->y_p /= h;
+    der->x_p /= h;
+    der->y_l /= h;
+
+    return 0;
+}