medal-solvers/task255_solver_265.py

"""Task 255 solver — 265/265 pass (ALL examples correct).

Algorithm:
1. Find largest empty rect → erode non-boundary sides → core
2. PHASE 1: RIGHT/LEFT extensions (single pass, dynamic eligibility from core)
3. PHASE 2: Sub-rect post-processing for runs with wide gaps (static eligibility, gap >= 4)
4. PHASE 3: DOWN/UP extensions (height>=3 guard + adjacent no-fg col extension)
5. PHASE 4: Second RIGHT/LEFT pass (width>=3 guard, cascades from UP/DOWN vertical arms)
6. Mask to background (inp==0) only

Key insights that solved all 4 previously-failing cases:
- [116],[218]: Sub-rect in static-eligible rows extends >= 4 beyond main rect → use eroded sub-rect boundary
- [155],[173]: height>=3 guard on DOWN/UP prevents short RIGHT/LEFT arms from cascading perpendicular
- [130]: Adjacent no-fg cols extend the UP/DOWN eligible run; second RIGHT/LEFT pass handles cascade from vertical arm

Validated: 3/3 train + 1/1 test + 261/261 arc-gen = 265/265 PASS
"""
import numpy as np
import json


def find_largest_empty_rect(inp):
    h, w = inp.shape
    heights = np.zeros(w, dtype=int)
    best_area, best = 0, (0, 0, 0, 0)
    for r in range(h):
        for c in range(w):
            heights[c] = heights[c] + 1 if inp[r, c] == 0 else 0
        stack = []
        for c in range(w + 1):
            cur_h = heights[c] if c < w else 0
            start = c
            while stack and stack[-1][1] > cur_h:
                idx, sh = stack.pop()
                area = sh * (c - idx)
                if area > best_area:
                    best_area = area
                    best = (r - sh + 1, idx, sh, c - idx)
                start = idx
            if not stack or cur_h > stack[-1][1]:
                stack.append((start, cur_h))
    return best


def erode_range(start, end, grid_max):
    s, e = start, end
    if s > 0: s += 1
    if e < grid_max: e -= 1
    return s, e


def find_contiguous_runs(indices):
    if not indices: return []
    runs, start, prev = [], indices[0], indices[0]
    for i in indices[1:]:
        if i == prev + 1: prev = i
        else: runs.append((start, prev)); start = prev = i
    runs.append((start, prev))
    return runs


def apply_rule(inp):
    h, w = inp.shape
    tr, lc, rh, rw = find_largest_empty_rect(inp)
    main_r1, main_r2 = tr, tr + rh - 1
    main_c1, main_c2 = lc, lc + rw - 1
    core_r1, core_r2 = erode_range(main_r1, main_r2, h - 1)
    core_c1, core_c2 = erode_range(main_c1, main_c2, w - 1)
    
    result = np.zeros((h, w), dtype=bool)
    result[core_r1:core_r2+1, core_c1:core_c2+1] = True
    
    # PHASE 1: RIGHT/LEFT (dynamic eligibility, single pass)
    right_elig = []
    for r in range(h):
        rc = np.where(result[r])[0]
        if len(rc) == 0: continue
        fg = np.where(inp[r] != 0)[0]
        if len(fg) == 0 or fg[-1] < rc[0]:
            right_elig.append(r)
    for rs, re in find_contiguous_runs(sorted(right_elig)):
        es, ee = erode_range(rs, re, h - 1)
        if es <= ee:
            for r in range(es, ee + 1):
                rc = np.where(result[r])[0]
                if len(rc) > 0: result[r, rc[-1]+1:] = True

    left_elig = []
    for r in range(h):
        rc = np.where(result[r])[0]
        if len(rc) == 0: continue
        fg = np.where(inp[r] != 0)[0]
        if len(fg) == 0 or fg[0] > rc[-1]:
            left_elig.append(r)
    for rs, re in find_contiguous_runs(sorted(left_elig)):
        es, ee = erode_range(rs, re, h - 1)
        if es <= ee:
            for r in range(es, ee + 1):
                rc = np.where(result[r])[0]
                if len(rc) > 0: result[r, :rc[0]] = True

    # PHASE 2: Sub-rect post-processing (static eligibility, threshold >= 4)
    static_right = []
    for r in range(h):
        fg = np.where(inp[r] != 0)[0]
        if len(fg) == 0 or fg[-1] < core_c1:
            static_right.append(r)
    for rs, re in find_contiguous_runs(sorted(static_right)):
        sub = inp[rs:re+1, :]
        sub_tr, sub_lc, sub_rh, sub_rw = find_largest_empty_rect(sub)
        if main_c1 - sub_lc >= 4:
            e_r1, e_r2 = erode_range(rs + sub_tr, rs + sub_tr + sub_rh - 1, h - 1)
            e_c1, _ = erode_range(sub_lc, sub_lc + sub_rw - 1, w - 1)
            if e_r1 <= e_r2:
                for r in range(e_r1, e_r2 + 1):
                    result[r, e_c1:] = True

    static_left = []
    for r in range(h):
        fg = np.where(inp[r] != 0)[0]
        if len(fg) == 0 or fg[0] > core_c2:
            static_left.append(r)
    for rs, re in find_contiguous_runs(sorted(static_left)):
        sub = inp[rs:re+1, :]
        sub_tr, sub_lc, sub_rh, sub_rw = find_largest_empty_rect(sub)
        sub_c2 = sub_lc + sub_rw - 1
        if sub_c2 - main_c2 >= 4:
            e_r1, e_r2 = erode_range(rs + sub_tr, rs + sub_tr + sub_rh - 1, h - 1)
            _, e_c2 = erode_range(sub_lc, sub_c2, w - 1)
            if e_r1 <= e_r2:
                for r in range(e_r1, e_r2 + 1):
                    result[r, :e_c2+1] = True

    # PHASE 3: DOWN/UP (dynamic, height>=3 guard + adjacent no-fg col extension)
    down_elig = set()
    for c in range(w):
        rr = np.where(result[:, c])[0]
        if len(rr) == 0: continue
        if (rr[-1] - rr[0] + 1) < 3: continue
        fg = np.where(inp[:, c] != 0)[0]
        if len(fg) == 0 or fg[-1] < rr[0]:
            down_elig.add(c)
    extended = set(down_elig)
    for c in sorted(down_elig):
        for adj in [c-1, c+1]:
            if 0 <= adj < w and adj not in extended:
                fg_adj = np.where(inp[:, adj] != 0)[0]
                if len(fg_adj) == 0:
                    extended.add(adj)
    for cs, ce in find_contiguous_runs(sorted(extended)):
        es, ee = erode_range(cs, ce, w - 1)
        if es <= ee:
            for c in range(es, ee + 1):
                rr = np.where(result[:, c])[0]
                if len(rr) > 0:
                    result[rr[-1]+1:, c] = True
                else:
                    result[core_r2+1:, c] = True

    up_elig = set()
    for c in range(w):
        rr = np.where(result[:, c])[0]
        if len(rr) == 0: continue
        if (rr[-1] - rr[0] + 1) < 3: continue
        fg = np.where(inp[:, c] != 0)[0]
        if len(fg) == 0 or fg[0] > rr[-1]:
            up_elig.add(c)
    extended_up = set(up_elig)
    for c in sorted(up_elig):
        for adj in [c-1, c+1]:
            if 0 <= adj < w and adj not in extended_up:
                fg_adj = np.where(inp[:, adj] != 0)[0]
                if len(fg_adj) == 0:
                    extended_up.add(adj)
    for cs, ce in find_contiguous_runs(sorted(extended_up)):
        es, ee = erode_range(cs, ce, w - 1)
        if es <= ee:
            for c in range(es, ee + 1):
                rr = np.where(result[:, c])[0]
                if len(rr) > 0:
                    result[:rr[0], c] = True
                else:
                    result[:core_r1, c] = True

    # PHASE 4: Second RIGHT/LEFT pass (width>=3 guard, after UP/DOWN)
    right_elig2 = []
    for r in range(h):
        rc = np.where(result[r])[0]
        if len(rc) == 0: continue
        if (rc[-1] - rc[0] + 1) < 3: continue
        fg = np.where(inp[r] != 0)[0]
        if len(fg) == 0 or fg[-1] < rc[0]:
            right_elig2.append(r)
    for rs, re in find_contiguous_runs(sorted(right_elig2)):
        es, ee = erode_range(rs, re, h - 1)
        if es <= ee:
            for r in range(es, ee + 1):
                rc = np.where(result[r])[0]
                if len(rc) > 0: result[r, rc[-1]+1:] = True

    left_elig2 = []
    for r in range(h):
        rc = np.where(result[r])[0]
        if len(rc) == 0: continue
        if (rc[-1] - rc[0] + 1) < 3: continue
        fg = np.where(inp[r] != 0)[0]
        if len(fg) == 0 or fg[0] > rc[-1]:
            left_elig2.append(r)
    for rs, re in find_contiguous_runs(sorted(left_elig2)):
        es, ee = erode_range(rs, re, h - 1)
        if es <= ee:
            for r in range(es, ee + 1):
                rc = np.where(result[r])[0]
                if len(rc) > 0: result[r, :rc[0]] = True

    return result & (inp == 0)


def verify_task255(task_data_path='task-data/task255.json'):
    with open(task_data_path) as f:
        data = json.load(f)
    total_pass, total_fail, failures = 0, 0, []
    for split in ['train', 'test', 'arc-gen']:
        for idx, ex in enumerate(data[split]):
            inp_arr = np.array(ex['input'], dtype=int)
            exp = np.array(ex['output'], dtype=int)
            mask = apply_rule(inp_arr)
            pred = inp_arr.copy(); pred[mask] = 3
            if np.array_equal(pred, exp):
                total_pass += 1
            else:
                total_fail += 1
                m = ((exp == 3) & (pred != 3)).sum()
                e = ((pred == 3) & (exp != 3)).sum()
                failures.append({'split': split, 'idx': idx, 'missing': int(m), 'extra': int(e)})
    print(f"Results: {total_pass} pass, {total_fail} fail")
    for f in failures[:15]:
        print(f"  {f['split']}[{f['idx']}]: M={f['missing']}, E={f['extra']}")
    return total_pass, total_fail, failures


if __name__ == '__main__':
    verify_task255()