//
// Created by david on 04.03.2026.
//

#include "include/pd_signal.h"
#include <stdexcept>
#include <algorithm>

namespace pd_signal {

static void throwIfNotAscending(std::vector<double>& xp) {
    size_t N = xp.size();
    for(int i = 0; i < N - 1; i++)
        if((xp[i+1] - xp[i]) <= 0.0)
            throw std::invalid_argument("xp is not strictly monotonically ascending");
}

static int binarySearch(std::vector<double>& data, double x) {
    auto p = std::equal_range(data.begin(), data.end(), x);
    if(p.first == data.end()) return -((int) data.size()) - 1; // out of range on upper end
    int insertion_point = (int) std::distance(data.begin(), p.first);
    if(p.first != p.second) return insertion_point; // element found
    return -(insertion_point) - 1; // no element found directly
}

// Evaluate at points x the function given by the samples fp[xp[n]].
void interp(std::vector<double>& x, std::vector<double>& xp, std::vector<double>& fp, std::vector<double>& y) {
    if (xp.size() != fp.size()) throw std::invalid_argument("xp.size() != fp.size()");
    size_t N = xp.size();
    size_t M = x.size();
    if (N < 2) throw std::invalid_argument("N < 2");
    throwIfNotAscending(xp);

    std::vector<double> slope, intercept;
    y.resize(M);
    slope.resize(N-1);
    intercept.resize(N-1);

    for(int i = 0; i < N-1; i++) {
        double dxp = xp[i+1] - xp[i];
        double dfp = fp[i+1] - fp[i];
        slope[i] = dfp / dxp;
        intercept[i] = fp[i] - slope[i] * xp[i];
    }

    for(int i = 0; i < M; i++) {
        if(x[i] < xp[0]) {
            y[i] = fp[0];
        } else if(x[i] > xp[N-1]) {
            y[i] = fp[N-1];
        } else {
            int idx = binarySearch(xp, x[i]);
            if (idx <= -1) {
                // not found precisely. interpolate
                idx = -idx - 1; // convert back to array index (insertion point)
                // idx==0 or idx==M cannot happen, handled above.
                //assert(idx > 0 && idx < N);
                idx = idx - 1; // one below insertion point (left point)
                y[i] = slope[idx] * x[i] + intercept[idx];
            } else {
                // found precisely
                y[i] = fp[idx];
            }
        }
    }
}

}