#include "game/build_graph.hpp"
#include <cmath>
#include <cstdio>
#include <algorithm>

namespace {
constexpr float PI = 3.14159265358979323846f;
uint64_t edge_key(int a, int b) {
    if (a > b) std::swap(a, b);
    return (uint64_t)(uint32_t)a << 32 | (uint32_t)b;
}
float wrap_angle(float d) {
    while (d >  PI) d -= 2.0f * PI;
    while (d < -PI) d += 2.0f * PI;
    return d;
}
}

// =============================================================================
// Construction
// =============================================================================

int BuildGraph::add_node(float x, float y, bool foundation) {
    nodes.push_back({x, y, 0.0f, 0.0f, MIN_NODE_MASS, foundation});
    return (int)nodes.size() - 1;
}

int BuildGraph::add_edge(int na, int nb, int mat) {
    if (na < 0 || na >= (int)nodes.size()) return -1;
    if (nb < 0 || nb >= (int)nodes.size()) return -1;
    if (na == nb || edge_exists(na, nb)) return -1;
    if (materials.empty()) return -1;
    if (mat < 0 || mat >= (int)materials.size()) mat = 0;

    const BuildNode& A = nodes[na];
    const BuildNode& B = nodes[nb];
    float dx = B.x - A.x, dy = B.y - A.y;
    const MaterialDef& m = materials[mat];

    BuildEdge e{};
    e.node_a      = na;
    e.node_b      = nb;
    e.mat         = mat;
    e.rest_length = std::sqrt(dx*dx + dy*dy);
    e.rest_angle  = std::atan2(dy, dx);
    e.stress      = 0.0f;
    e.hp = e.max_hp = m.hit_points;
    e.burning     = false;
    e.burn        = 0.0f;
    e.settle      = m.tension_only ? 0.0f : SETTLE_TIME;  // ropes are floppy by nature
    e.half_width  = m.half_width;
    e.r = m.r; e.g = m.g; e.b = m.b; e.a = m.a;
    edges.push_back(e);

    rebuild_topology();
    return (int)edges.size() - 1;
}

int BuildGraph::add_link(int na, int nb, int mat) {
    if (na < 0 || na >= (int)nodes.size()) return 0;
    if (nb < 0 || nb >= (int)nodes.size()) return 0;
    if (na == nb || materials.empty()) return 0;
    if (mat < 0 || mat >= (int)materials.size()) mat = 0;
    const MaterialDef& m = materials[mat];

    float ax = nodes[na].x, ay = nodes[na].y;
    float bx = nodes[nb].x, by = nodes[nb].y;
    float dist = std::hypot(bx - ax, by - ay);
    if (dist < m.min_length || dist > m.max_link_length) return 0;

    int segs = (m.max_length > 0.0f) ? (int)std::ceil(dist / m.max_length) : 1;
    if (segs < 1) segs = 1;
    if (segs == 1) return add_edge(na, nb, mat) >= 0 ? 1 : 0;

    // Subdivide into a chain of intermediate nodes so no segment exceeds max_length.
    int created = 0, prev = na;
    for (int i = 1; i < segs; i++) {
        float t = (float)i / segs;
        float px = ax + (bx - ax) * t, py = ay + (by - ay) * t;
        int mid = add_node(px, py, is_on_ground(py));
        if (add_edge(prev, mid, mat) >= 0) created++;
        prev = mid;
    }
    if (add_edge(prev, nb, mat) >= 0) created++;
    return created;
}

void BuildGraph::extrude_edge(int edge_id, float off_x, float off_y) {
    if (edge_id < 0 || edge_id >= (int)edges.size()) return;

    int mat0 = edges[edge_id].mat;
    if (mat0 >= 0 && mat0 < (int)materials.size()) {
        // Keep box sides single (braceable) edges: clamp the drag to max_length.
        float ol = std::sqrt(off_x*off_x + off_y*off_y);
        float maxlen = materials[mat0].max_length;
        if (ol > maxlen && ol > 1e-6f) { off_x *= maxlen / ol; off_y *= maxlen / ol; }
    }
    if (std::sqrt(off_x*off_x + off_y*off_y) < 0.1f) return;

    const BuildEdge e = edges[edge_id];          // snapshot before vectors move
    const int   ea = e.node_a, eb = e.node_b, mat = e.mat;
    const float ax = nodes[ea].x, ay = nodes[ea].y;
    const float bx = nodes[eb].x, by = nodes[eb].y;

    float cx = ax + off_x, cy = ay + off_y;      // slanted parallelogram is fine
    float dx = bx + off_x, dy = by + off_y;

    // Node merging: reuse a nearby existing node so the box grafts into the graph.
    int nc = find_nearest_node(cx, cy, SNAP_RADIUS);
    if (nc < 0 || nc == ea || nc == eb) nc = add_node(cx, cy, is_on_ground(cy));
    int nd = find_nearest_node(dx, dy, SNAP_RADIUS);
    if (nd < 0 || nd == ea || nd == eb || nd == nc) nd = add_node(dx, dy, is_on_ground(dy));

    add_edge(nc, nd, mat);   // three new sides (original edge is the fourth)
    add_edge(ea, nc, mat);
    add_edge(eb, nd, mat);

    // Auto diagonal brace (longer diagonal) so the box is rigid on creation.
    if (!materials[mat].tension_only) {
        float l1 = std::hypot(nodes[nd].x - nodes[ea].x, nodes[nd].y - nodes[ea].y);
        float l2 = std::hypot(nodes[nc].x - nodes[eb].x, nodes[nc].y - nodes[eb].y);
        if (l1 >= l2) { if (l1 >= MIN_BRACE_LENGTH) add_edge(ea, nd, mat); }
        else          { if (l2 >= MIN_BRACE_LENGTH) add_edge(eb, nc, mat); }
    }
}

// =============================================================================
// Topology — adjacency, edge lookup, faces, triangulation, node mass  O(E.deg)
// =============================================================================

void BuildGraph::rebuild_topology() {
    adj_.assign(nodes.size(), {});
    edge_set_.clear();
    for (int ei = 0; ei < (int)edges.size(); ei++) {
        const auto& e = edges[ei];
        adj_[e.node_a].push_back({e.node_b, ei});
        adj_[e.node_b].push_back({e.node_a, ei});
        edge_set_.insert(edge_key(e.node_a, e.node_b));
    }

    // Faces + per-edge triangulated flag (common neighbour of both endpoints).
    faces.clear();
    for (auto& e : edges) {
        int a = std::min(e.node_a, e.node_b);
        int b = std::max(e.node_a, e.node_b);
        bool tri = false;
        for (auto [c, _] : adj_[a]) {
            if (c == b) continue;
            if (edge_set_.count(edge_key(b, c))) {
                tri = true;
                if (c > b) faces.push_back({a, b, c});  // emit each triangle once
            }
        }
        e.triangulated = tri;
    }

    // Node mass = half of each incident strut's mass, floored.
    for (auto& n : nodes) n.mass = 0.0f;
    for (const auto& e : edges) {
        float half = materials[e.mat].mass * 0.5f;
        nodes[e.node_a].mass += half;
        nodes[e.node_b].mass += half;
    }
    for (auto& n : nodes) if (n.mass < MIN_NODE_MASS) n.mass = MIN_NODE_MASS;
}

bool BuildGraph::edge_exists(int a, int b) const {
    return edge_set_.count(edge_key(a, b)) != 0;
}

// =============================================================================
// Simulation — stiff damped mass-spring, integrated with oversampling
// =============================================================================

void BuildGraph::step(float dt) {
    const int   n = (int)nodes.size();
    const float h = dt / OVERSAMPLES;
    float dampf = 1.0f - air_drag * h;
    if (dampf < 0.0f) dampf = 0.0f;

    // Fresh struts are held rigid during their build grace: freeze their nodes.
    held_.assign(n, 0);
    for (const auto& e : edges) {
        if (e.settle <= 0.0f) continue;
        if (!nodes[e.node_a].is_foundation) held_[e.node_a] = 1;
        if (!nodes[e.node_b].is_foundation) held_[e.node_b] = 1;
    }

    for (int s = 0; s < OVERSAMPLES; s++) {
        fx_.assign(n, 0.0f);
        fy_.assign(n, 0.0f);

        // gravity (held/pinned nodes don't move, so skip)
        for (int i = 0; i < n; i++)
            if (!nodes[i].is_foundation && !held_[i])
                fy_[i] = -gravity * nodes[i].mass;

        // spring forces: F = k*stretch + c*(relative velocity along axis)
        for (const auto& e : edges) {
            BuildNode& A = nodes[e.node_a];
            BuildNode& B = nodes[e.node_b];
            float dx = B.x - A.x, dy = B.y - A.y;
            float dist = std::sqrt(dx*dx + dy*dy);
            if (dist < 1e-6f) continue;
            float nx = dx / dist, ny = dy / dist;
            float stretch = dist - e.rest_length;
            const MaterialDef& m = materials[e.mat];
            if (m.tension_only && stretch < 0.0f) continue;   // rope slack

            float relv = (B.vx - A.vx) * nx + (B.vy - A.vy) * ny;
            float f = m.stiffness * stretch + m.damping * relv;
            fx_[e.node_a] += f * nx; fy_[e.node_a] += f * ny;
            fx_[e.node_b] -= f * nx; fy_[e.node_b] -= f * ny;
        }

        // semi-implicit Euler integration
        for (int i = 0; i < n; i++) {
            BuildNode& p = nodes[i];
            if (p.is_foundation || held_[i]) { p.vx = p.vy = 0.0f; continue; }
            p.vx = (p.vx + fx_[i] / p.mass * h) * dampf;
            p.vy = (p.vy + fy_[i] / p.mass * h) * dampf;
            p.x += p.vx * h;
            p.y += p.vy * h;
        }
    }

    // Record signed axial deformation for stress colouring.
    for (auto& e : edges) {
        float dx = nodes[e.node_b].x - nodes[e.node_a].x;
        float dy = nodes[e.node_b].y - nodes[e.node_a].y;
        float dist = std::sqrt(dx*dx + dy*dy);
        e.stress = (e.rest_length > 1e-6f) ? (dist - e.rest_length) / e.rest_length : 0.0f;
    }
}

int BuildGraph::check_strain() {
    std::vector<int> brk;
    for (int ei = 0; ei < (int)edges.size(); ei++) {
        const BuildEdge&   e = edges[ei];
        const MaterialDef& m = materials[e.mat];
        const BuildNode&   A = nodes[e.node_a];
        const BuildNode&   B = nodes[e.node_b];
        float dx = B.x - A.x, dy = B.y - A.y;
        float dist = std::sqrt(dx*dx + dy*dy);
        float ratio = (e.rest_length > 1e-6f) ? dist / e.rest_length : 1.0f;

        bool fail = false;
        if (m.tension_only) {
            fail = ratio > m.max_expansion;                   // rope: over-stretch only
        } else {
            fail = ratio < m.max_compression || ratio > m.max_expansion;
            // Angle stress: a loose (un-triangulated) strut past its grace snaps
            // once it rotates too far from the angle it was built at.
            if (!fail && !e.triangulated && e.settle <= 0.0f) {
                float dev = std::fabs(wrap_angle(std::atan2(dy, dx) - e.rest_angle));
                if (dev > m.angle_threshold) fail = true;
            }
        }
        if (fail) brk.push_back(ei);
    }
    if (brk.empty()) return 0;

    for (auto it = brk.rbegin(); it != brk.rend(); ++it)  // descending: indices stay valid
        edges.erase(edges.begin() + *it);
    rebuild_topology();
    printf("Build: %zu strut(s) snapped\n", brk.size());
    return (int)brk.size();
}

void BuildGraph::update_timers(float dt) {
    for (auto& e : edges)
        if (e.settle > 0.0f)
            e.settle = e.triangulated ? 0.0f : std::max(0.0f, e.settle - dt);
}

int BuildGraph::kill_grounded() {
    int killed = 0;
    for (;;) {
        int hit = -1;
        for (int i = 0; i < (int)nodes.size(); i++)
            if (!nodes[i].is_foundation && nodes[i].y < ground_level) { hit = i; break; }
        if (hit < 0) break;
        destroyed_events.push_back({nodes[hit].x, nodes[hit].y});
        break_node(hit);
        killed++;
    }
    return killed;
}

// =============================================================================
// Destruction
// =============================================================================

void BuildGraph::apply_recoil(float x, float y, float radius,
                              float dx, float dy, float strength) {
    if (radius <= 0.0f || strength <= 0.0f) return;

    for (auto& n : nodes) {
        if (n.is_foundation) continue;   // pinned: the ground eats the recoil
        float ox = n.x - x, oy = n.y - y;
        float d = std::sqrt(ox*ox + oy*oy);
        if (d >= radius) continue;
        float f = 1.0f - d / radius;     // linear falloff, as in apply_splash
        n.vx += dx * strength * f;
        n.vy += dy * strength * f;
    }
}

void BuildGraph::apply_splash(float x, float y, float radius, float damage,
                             float knockback) {
    if (radius <= 0.0f) return;

    // Knockback: shove nearby free nodes away from the blast (linear falloff).
    for (auto& n : nodes) {
        if (n.is_foundation) continue;
        float dx = n.x - x, dy = n.y - y;
        float d = std::sqrt(dx*dx + dy*dy);
        if (d >= radius) continue;
        float f = 1.0f - d / radius;
        if (d > 1e-4f) { n.vx += (dx/d) * knockback * f; n.vy += (dy/d) * knockback * f; }
    }

    // Damage struts by their midpoint distance; collect those that hit 0 HP.
    std::vector<int> brk;
    for (int ei = 0; ei < (int)edges.size(); ei++) {
        auto& e = edges[ei];
        float mx = (nodes[e.node_a].x + nodes[e.node_b].x) * 0.5f;
        float my = (nodes[e.node_a].y + nodes[e.node_b].y) * 0.5f;
        float d = std::sqrt((mx-x)*(mx-x) + (my-y)*(my-y));
        if (d >= radius) continue;
        e.hp -= damage * (1.0f - d / radius);
        if (e.hp <= 0.0f) brk.push_back(ei);
    }
    if (!brk.empty()) {
        for (auto it = brk.rbegin(); it != brk.rend(); ++it)
            edges.erase(edges.begin() + *it);
        rebuild_topology();
    }

    // Splash also damages devices mounted within the radius.
    for (auto& dv : devices) {
        if (!dv.alive) continue;
        float mx = 0.0f, my = 0.0f; int mounts = 0;
        if (dv.node_a >= 0 && dv.node_a < (int)nodes.size())
            { mx += nodes[dv.node_a].x; my += nodes[dv.node_a].y; mounts++; }
        if (dv.node_b >= 0 && dv.node_b < (int)nodes.size())
            { mx += nodes[dv.node_b].x; my += nodes[dv.node_b].y; mounts++; }
        if (mounts == 0) continue;
        mx /= mounts; my /= mounts;
        float d = std::sqrt((mx-x)*(mx-x) + (my-y)*(my-y));
        if (d < radius) { dv.hp -= damage * (1.0f - d / radius); if (dv.hp <= 0.0f) dv.alive = false; }
    }
}

int BuildGraph::find_blocking_edge(float x, float y, float radius) const {
    int best = -1; float best_d = radius;
    for (int i = 0; i < (int)edges.size(); i++) {
        if (!materials[edges[i].mat].blocks_projectiles) continue;
        const BuildNode& A = nodes[edges[i].node_a];
        const BuildNode& B = nodes[edges[i].node_b];
        float ex = B.x - A.x, ey = B.y - A.y;
        float len2 = ex*ex + ey*ey;
        if (len2 < 1e-6f) continue;
        float t = std::clamp(((x-A.x)*ex + (y-A.y)*ey) / len2, 0.0f, 1.0f);
        float d = std::hypot(x - (A.x + t*ex), y - (A.y + t*ey));
        if (d < best_d) { best_d = d; best = i; }
    }
    return best;
}

float BuildGraph::beam_fire(float ox, float oy, float dx, float dy, float range,
                            float damage, bool ignite, float& hit_x, float& hit_y) {
    float dl = std::hypot(dx, dy);
    if (dl < 1e-6f) { hit_x = ox; hit_y = oy; return 0.0f; }
    dx /= dl; dy /= dl;

    // Gather every strut the ray crosses (of any material), sorted by distance.
    struct Cross { float t; int edge; };
    std::vector<Cross> crosses;
    for (int i = 0; i < (int)edges.size(); i++) {
        const BuildNode& A = nodes[edges[i].node_a];
        const BuildNode& B = nodes[edges[i].node_b];
        float ex = B.x - A.x, ey = B.y - A.y;
        float denom = dx * ey - dy * ex;
        if (std::fabs(denom) < 1e-6f) {
            // Ray parallel to segment. Check for collinear overlap.
            float perp = std::fabs((ox - A.x) * ey - (oy - A.y) * ex);
            if (perp > 1e-4f) continue;                     // parallel, no overlap
            float ta = (A.x - ox) * dx + (A.y - oy) * dy;
            float tb = (B.x - ox) * dx + (B.y - oy) * dy;
            if (ta > tb) std::swap(ta, tb);
            if (tb < 0.0f || ta > range) continue;          // no overlap
            crosses.push_back({std::max(0.0f, ta), i});
            continue;
        }
        float t = ((A.x - ox) * ey - (A.y - oy) * ex) / denom; // dist along ray
        float u = ((A.x - ox) * dy - (A.y - oy) * dx) / denom; // param along segment
        if (t >= 0.0f && t <= range && u >= 0.0f && u <= 1.0f) crosses.push_back({t, i});
    }
    std::sort(crosses.begin(), crosses.end(),
              [](const Cross& a, const Cross& b){ return a.t < b.t; });

    // Damage/ignite each crossed strut; pass through transparent ones; stop at wood.
    float stop = range;
    std::vector<int> brk;
    for (const auto& c : crosses) {
        BuildEdge& e = edges[c.edge];
        e.hp -= damage;
        if (ignite && materials[e.mat].flammable) e.burning = true;
        if (e.hp <= 0.0f) brk.push_back(c.edge);
        if (materials[e.mat].blocks_beam) { stop = c.t; break; }   // wood halts the beam
    }
    hit_x = ox + dx * stop;
    hit_y = oy + dy * stop;

    if (!brk.empty()) {
        std::sort(brk.begin(), brk.end());
        brk.erase(std::unique(brk.begin(), brk.end()), brk.end());
        for (auto it = brk.rbegin(); it != brk.rend(); ++it)
            edges.erase(edges.begin() + *it);
        rebuild_topology();
    }

    // Beam also damages devices within threshold distance of its path.
    {
        float ex = hit_x - ox, ey = hit_y - oy;
        float len2 = ex*ex + ey*ey;
        if (len2 > 1e-6f) {
            for (auto& dv : devices) {
                if (!dv.alive) continue;
                float mx = 0.0f, my = 0.0f; int mounts = 0;
                if (dv.node_a >= 0 && dv.node_a < (int)nodes.size())
                    { mx += nodes[dv.node_a].x; my += nodes[dv.node_a].y; mounts++; }
                if (dv.node_b >= 0 && dv.node_b < (int)nodes.size())
                    { mx += nodes[dv.node_b].x; my += nodes[dv.node_b].y; mounts++; }
                if (mounts == 0) continue;
                mx /= mounts; my /= mounts;
                float t = std::clamp(((mx-ox)*ex + (my-oy)*ey) / len2, 0.0f, 1.0f);
                float d = std::hypot(mx - (ox + t*ex), my - (oy + t*ey));
                if (d < 0.6f) { dv.hp -= damage; if (dv.hp <= 0.0f) dv.alive = false; }
            }
        }
    }

    return stop;
}

void BuildGraph::ignite_edge(int edge_id) {
    if (edge_id < 0 || edge_id >= (int)edges.size()) return;
    if (materials[edges[edge_id].mat].flammable) edges[edge_id].burning = true;
}

void BuildGraph::ignite_area(float x, float y, float radius) {
    for (int i = 0; i < (int)edges.size(); i++) {
        float mx = (nodes[edges[i].node_a].x + nodes[edges[i].node_b].x) * 0.5f;
        float my = (nodes[edges[i].node_a].y + nodes[edges[i].node_b].y) * 0.5f;
        if (std::hypot(mx - x, my - y) < radius) ignite_edge(i);
    }
}

void BuildGraph::update_fire(float dt) {
    // Burn: DoT + advance spread timer; ignite flammable neighbours; destroy at 0.
    std::vector<int> newly_lit;
    std::vector<int> brk;
    for (int i = 0; i < (int)edges.size(); i++) {
        auto& e = edges[i];
        if (!e.burning) continue;
        const MaterialDef& m = materials[e.mat];
        e.hp  -= m.burn_rate * dt;
        e.burn += dt;
        if (e.burn >= m.spread_time) {
            e.burn = 0.0f;   // spread again after each interval
            for (int end : { e.node_a, e.node_b })
                for (auto [nb, ei] : adj_[end])
                    if (!edges[ei].burning && materials[edges[ei].mat].flammable)
                        newly_lit.push_back(ei);
        }
        if (e.hp <= 0.0f) brk.push_back(i);
    }
    for (int ei : newly_lit)
        if (ei >= 0 && ei < (int)edges.size()) edges[ei].burning = true;
    if (!brk.empty()) {
        std::sort(brk.begin(), brk.end());
        brk.erase(std::unique(brk.begin(), brk.end()), brk.end());
        for (auto it = brk.rbegin(); it != brk.rend(); ++it)
            edges.erase(edges.begin() + *it);
        rebuild_topology();
    }

    // Fire also damages devices mounted on burning struts.
    for (int ei = 0; ei < (int)edges.size(); ei++) {
        const auto& e = edges[ei];
        if (!e.burning) continue;
        float fd = materials[e.mat].burn_rate * dt;
        for (auto& dv : devices) {
            if (!dv.alive) continue;
            if (dv.node_a == e.node_a || dv.node_a == e.node_b ||
                dv.node_b == e.node_a || dv.node_b == e.node_b) {
                dv.hp -= fd; if (dv.hp <= 0.0f) dv.alive = false;
            }
        }
    }
}

void BuildGraph::break_edge(int edge_id) {
    if (edge_id < 0 || edge_id >= (int)edges.size()) return;
    edges.erase(edges.begin() + edge_id);
    rebuild_topology();
}

void BuildGraph::break_node(int node_id) {
    if (node_id < 0 || node_id >= (int)nodes.size()) return;

    edges.erase(std::remove_if(edges.begin(), edges.end(),
        [node_id](const BuildEdge& e) {
            return e.node_a == node_id || e.node_b == node_id;
        }), edges.end());

    nodes.erase(nodes.begin() + node_id);
    for (auto& e : edges) {
        if (e.node_a > node_id) e.node_a--;
        if (e.node_b > node_id) e.node_b--;
    }

    // Fix up device mount indices: invalidate mounts on the removed node,
    // shift indices past it, and mark devices dead when both mounts are gone.
    for (auto& dv : devices) {
        if (!dv.alive) continue;
        if (dv.node_a == node_id) dv.node_a = -1;
        if (dv.node_b == node_id) dv.node_b = -1;
        if (dv.node_a > node_id) dv.node_a--;
        if (dv.node_b > node_id) dv.node_b--;
        if (dv.node_a < 0 && dv.node_b < 0) dv.alive = false;
    }

    rebuild_topology();
}

// =============================================================================
// Devices mounted on the strut graph
// =============================================================================

int BuildGraph::mount_device(int na, int nb, int device_type, int team, float hp) {
    if (na < 0 || na >= (int)nodes.size()) return -1;
    if (nb < 0 || nb >= (int)nodes.size()) return -1;
    if (na == nb) return -1;

    MountedDevice md{};
    md.node_a = na;
    md.node_b = nb;
    md.type   = device_type;
    md.team   = team;
    md.hp     = hp;
    md.max_hp = hp;
    md.alive  = true;
    devices.push_back(md);
    return (int)devices.size() - 1;
}

void BuildGraph::unmount_device(int device_id) {
    if (device_id < 0 || device_id >= (int)devices.size()) return;
    devices[device_id].alive = false;
}

bool BuildGraph::is_device_alive(int device_id) const {
    if (device_id < 0 || device_id >= (int)devices.size()) return false;
    return devices[device_id].alive && devices[device_id].hp > 0.0f;
}

float BuildGraph::get_device_hp(int device_id) const {
    if (device_id < 0 || device_id >= (int)devices.size()) return 0.0f;
    return devices[device_id].hp;
}

bool BuildGraph::damage_device(int device_id, float amount) {
    if (device_id < 0 || device_id >= (int)devices.size()) return false;
    auto& dv = devices[device_id];
    if (!dv.alive || dv.hp <= 0.0f) return false;
    dv.hp -= amount;
    if (dv.hp <= 0.0f) {
        dv.alive = false;
        return true;
    }
    return false;
}

int BuildGraph::nearest_device(float x, float y, float radius) const {
    int   best = -1;
    float best_d2 = radius * radius;
    for (int i = 0; i < (int)devices.size(); i++) {
        if (!devices[i].alive || devices[i].hp <= 0.0f) continue;
        // Device position = midpoint of its valid mount nodes.
        float mx = 0.0f, my = 0.0f;
        int   mounts = 0;
        if (devices[i].node_a >= 0 && devices[i].node_a < (int)nodes.size()) {
            mx += nodes[devices[i].node_a].x;
            my += nodes[devices[i].node_a].y;
            mounts++;
        }
        if (devices[i].node_b >= 0 && devices[i].node_b < (int)nodes.size()) {
            mx += nodes[devices[i].node_b].x;
            my += nodes[devices[i].node_b].y;
            mounts++;
        }
        if (mounts == 0) continue;
        mx /= mounts; my /= mounts;
        float dx = mx - x, dy = my - y;
        float d2 = dx*dx + dy*dy;
        if (d2 < best_d2) { best_d2 = d2; best = i; }
    }
    return best;
}

// =============================================================================
// Queries
// =============================================================================

int BuildGraph::find_nearest_node(float x, float y, float radius) const {
    int   best = -1;
    float best_d2 = radius * radius;
    for (int i = 0; i < (int)nodes.size(); i++) {
        float dx = nodes[i].x - x, dy = nodes[i].y - y;
        float d2 = dx*dx + dy*dy;
        if (d2 < best_d2) { best_d2 = d2; best = i; }
    }
    return best;
}

int BuildGraph::find_nearest_edge(float x, float y, float radius) const {
    int   best = -1;
    float best_d = radius;
    for (int i = 0; i < (int)edges.size(); i++) {
        const BuildNode& A = nodes[edges[i].node_a];
        const BuildNode& B = nodes[edges[i].node_b];
        float ex = B.x - A.x, ey = B.y - A.y;
        float len2 = ex*ex + ey*ey;
        if (len2 < 0.0001f) continue;
        float t = ((x - A.x)*ex + (y - A.y)*ey) / len2;
        t = std::clamp(t, 0.0f, 1.0f);
        float px = A.x + t*ex, py = A.y + t*ey;
        float d = std::hypot(x - px, y - py);
        if (d < best_d) { best_d = d; best = i; }
    }
    return best;
}

BuildEdge* BuildGraph::mutable_edge(int id) {
    if (id < 0 || id >= (int)edges.size()) return nullptr;
    return &edges[id];
}