interp: add 2D Lagrangian interpolation in C

1 files changed, 59 insertions, 0 deletions

diff --git a/_interp_c_template.c b/_interp_c_template.c
new file mode 100644
index 0000000..e1c5f8a
--- /dev/null
+++ b/_interp_c_template.c
@@ -0,0 +1,59 @@
+
+#define JOIN2(a, b) a ## _ ## b
+#define FUNC2(name, stencil) JOIN2(name, stencil)
+#define FUNC(name) FUNC2(name, STENCIL)
+
+void FUNC(interp2d)(const double *src_start, const double *src_step,
+                    const double *src_val,
+                    const int *src_stride, const int *src_len,
+                    int nb_points, const double *x_vals, const double *y_vals,
+                    double *dst)
+{
+    for (int pt = 0; pt < nb_points; pt++) {
+        const double x = x_vals[pt];
+        const double y = y_vals[pt];
+
+        const int    idx_x = (x - src_start[0]) / src_step[0] - (STENCIL / 2 - 1);
+        const int    idx_y = (y - src_start[1]) / src_step[1] - (STENCIL / 2 - 1);
+
+        const double src_x = idx_x * src_step[0] + src_start[0];
+        const double src_y = idx_y * src_step[1] + src_start[1];
+
+        double fact[2][STENCIL];
+
+        double ret = 0.0;
+
+        if (idx_x < 0 || idx_x > src_len[0] - STENCIL ||
+            idx_y < 0 || idx_y > src_len[1] - STENCIL) {
+            ret = NAN;
+            goto output;
+        }
+
+        for (int i = 0; i < STENCIL; i++) {
+            double fact_x = 1.0;
+            double fact_y = 1.0;
+
+            for (int j = 0; j < STENCIL; j++) {
+                if (i == j)
+                    continue;
+
+                fact_x *= (x - (src_x + j * src_step[0])) / (src_step[0] * (i - j));
+                fact_y *= (y - (src_y + j * src_step[1])) / (src_step[1] * (i - j));
+            }
+
+            fact[0][i] = fact_x;
+            fact[1][i] = fact_y;
+        }
+
+        for (int j = 0; j < STENCIL; j++) {
+            double val = 0.0;
+
+            for (int i = 0; i < STENCIL; i++)
+                val += fact[0][i] * src_val[(idx_y + j) * src_stride[1] + (idx_x + i) * src_stride[0]];
+
+            ret += fact[1][j] * val;
+        }
+output:
+        dst[pt] = ret;
+    }
+}