Initial commit: establish deterministic rod-string solver stack.

Set up the C solver core, Node API orchestration, TS GUI workflow, and engineering documentation with cleaned repo hygiene for private Git hosting.

Made-with: Cursor

solver-c/src/solver_fourier.c

@@ -0,0 +1,57 @@
+#include "solver.h"
+#include "solver_internal.h"
+
+#include <math.h>
+#include <string.h>
+
+#ifndef M_PI
+#define M_PI 3.14159265358979323846
+#endif
+
+static void reconstruct_series(const double *in, int n, int harmonics, double *out) {
+    if (n <= 0) return;
+    double a0 = 0.0;
+    for (int i = 0; i < n; i++) a0 += in[i];
+    a0 /= (double)n;
+    for (int i = 0; i < n; i++) out[i] = a0;
+
+    const int hmax = (harmonics > n / 2) ? (n / 2) : harmonics;
+    for (int k = 1; k <= hmax; k++) {
+        double ak = 0.0;
+        double bk = 0.0;
+        for (int i = 0; i < n; i++) {
+            const double th = 2.0 * M_PI * (double)k * (double)i / (double)n;
+            ak += in[i] * cos(th);
+            bk += in[i] * sin(th);
+        }
+        ak *= 2.0 / (double)n;
+        bk *= 2.0 / (double)n;
+        for (int i = 0; i < n; i++) {
+            const double th = 2.0 * M_PI * (double)k * (double)i / (double)n;
+            out[i] += ak * cos(th) + bk * sin(th);
+        }
+    }
+}
+
+int solver_compute_fourier_baseline(const SolverInputs *inputs, SolverOutputs *outputs) {
+    (void)inputs;
+    if (!outputs || outputs->point_count <= 0) return -1;
+    const int n = outputs->point_count;
+    const int harmonics = solver_clamp(inputs->fourier_harmonics, 1, SOLVER_MAX_FOURIER_HARMONICS);
+    reconstruct_series(outputs->polished_load, n, harmonics, outputs->fourier_polished_load);
+    reconstruct_series(outputs->downhole_load, n, harmonics, outputs->fourier_downhole_load);
+
+    double rss_p = 0.0;
+    double rss_d = 0.0;
+    for (int i = 0; i < n; i++) {
+        const double ep = outputs->polished_load[i] - outputs->fourier_polished_load[i];
+        const double ed = outputs->downhole_load[i] - outputs->fourier_downhole_load[i];
+        rss_p += ep * ep;
+        rss_d += ed * ed;
+    }
+    outputs->fourier_harmonics_used = harmonics;
+    outputs->fourier_residual_rms_polished = sqrt(rss_p / (double)n);
+    outputs->fourier_residual_rms_downhole = sqrt(rss_d / (double)n);
+    return 0;
+}
+