"""Applying a unified diff.

Pure unit tests, no server and no database: this is where the behaviour that
makes `file_edit` usable by a real model lives, and every case here is one that
a real model produces.
"""

from __future__ import annotations

import pytest

from lembas.services.agent import patch


def _apply(text: str, diff: str) -> str:
    return patch.apply(text, patch.parse(diff))


FILE = "\n".join(f"line {n}" for n in range(1, 21)) + "\n"


# --- The ordinary case ----------------------------------------------------------
def test_a_hunk_at_the_line_it_says_applies():
    result = _apply(
        FILE,
        "@@ -4,3 +4,3 @@\n line 3\n-line 4\n+LINE FOUR\n line 5\n",
    )
    assert "LINE FOUR" in result
    assert "line 4\n" not in result
    assert result.count("\n") == FILE.count("\n"), "no lines gained or lost"


def test_headers_are_tolerated():
    """Models emit them by habit. Refusing costs a round trip to say so."""
    result = _apply(
        FILE,
        "diff --git a/x.py b/x.py\nindex 1234567..89abcde 100644\n"
        "--- a/x.py\n+++ b/x.py\n@@ -4,3 +4,3 @@\n line 3\n-line 4\n+LINE FOUR\n line 5\n",
    )
    assert "LINE FOUR" in result


def test_several_hunks_apply_in_order():
    result = _apply(
        FILE,
        "@@ -2,3 +2,3 @@\n line 1\n-line 2\n+TWO\n line 3\n"
        "@@ -15,3 +15,3 @@\n line 14\n-line 15\n+FIFTEEN\n line 16\n",
    )
    assert "TWO" in result and "FIFTEEN" in result


def test_a_pure_insertion_needs_no_context():
    """And names the line it goes *after*, so it is not off by one the way
    every other hunk is."""
    result = _apply("a\nb\n", "@@ -1,0 +2,1 @@\n+inserted\n")
    assert result == "a\ninserted\nb\n"


# --- Line numbers drift, context does not ---------------------------------------
def test_a_hunk_whose_line_numbers_are_wrong_still_applies():
    """The single highest-value behaviour here. Models count from a truncated
    read or from the file as it was three edits ago and get the numbers wrong;
    they get the context right."""
    result = _apply(
        FILE,
        "@@ -1,3 +1,3 @@\n line 11\n-line 12\n+TWELVE\n line 13\n",
    )
    assert "TWELVE" in result
    assert "line 12\n" not in result


def test_a_hunk_that_matches_nowhere_is_refused_and_names_what_is_there():
    with pytest.raises(patch.PatchError) as caught:
        _apply(FILE, "@@ -4,3 +4,3 @@\n nothing\n-like this\n+new\n at all\n")

    message = caught.value.message
    assert "Hunk 1 did not apply" in message
    assert "Nothing was written" in message
    assert "Read the file again" in message


def test_ambiguous_context_is_refused_rather_than_guessed_at():
    """The one failure that silently corrupts a file. Two identical blocks and a
    hint pointing at neither: there is no way to tell which was meant."""
    text = "start\nsame\nsame\nsame\nmiddle\nsame\nsame\nsame\nend\n"
    with pytest.raises(patch.PatchError) as caught:
        _apply(text, "@@ -50,3 +50,3 @@\n same\n-same\n+CHANGED\n same\n")

    assert "appear" in caught.value.message
    assert "more unchanged lines" in caught.value.message.lower()


def test_drift_beyond_the_ceiling_is_not_searched():
    long = "\n".join(f"line {n}" for n in range(1, 1000)) + "\n"
    with pytest.raises(patch.PatchError):
        _apply(long, "@@ -1,3 +1,3 @@\n line 900\n-line 901\n+NINE\n line 902\n")


def test_nothing_is_written_when_a_later_hunk_fails():
    """Atomic. A half-applied file is worse than a refused one, and the model
    cannot tell the difference without reading it again."""
    with pytest.raises(patch.PatchError) as caught:
        _apply(
            FILE,
            "@@ -2,3 +2,3 @@\n line 1\n-line 2\n+TWO\n line 3\n"
            "@@ -15,3 +15,3 @@\n bogus\n-nope\n+x\n also bogus\n",
        )
    assert caught.value.hunk == 2


def test_hunks_out_of_order_are_refused():
    """Otherwise a duplicated hunk applies the same change twice."""
    with pytest.raises(patch.PatchError):
        _apply(
            FILE,
            "@@ -15,3 +15,3 @@\n line 14\n-line 15\n+FIFTEEN\n line 16\n"
            "@@ -2,3 +2,3 @@\n line 1\n-line 2\n+TWO\n line 3\n",
        )


# --- The things that break on real files ------------------------------------------
def test_a_crlf_file_round_trips_as_crlf():
    """Without normalising in and restoring out, every hunk on a Windows file
    fails on context that looks identical in the error message."""
    text = "alpha\r\nbeta\r\ngamma\r\n"
    result = _apply(text, "@@ -1,3 +1,3 @@\n alpha\n-beta\n+BETA\n gamma\n")

    assert result == "alpha\r\nBETA\r\ngamma\r\n"
    assert "\n\n" not in result.replace("\r\n", "\n\n").replace("\n\n", "\r\n")


def test_a_blank_context_line_with_no_leading_space_applies():
    """Trailing whitespace is stripped by half the things a model's output
    passes through, so this is the normal case rather than a malformed one."""
    text = "alpha\n\ngamma\n"
    result = _apply(text, "@@ -1,3 +1,3 @@\n alpha\n\n-gamma\n+GAMMA\n")
    assert result == "alpha\n\nGAMMA\n"


def test_a_file_with_no_trailing_newline_keeps_none():
    result = _apply("alpha\nbeta", "@@ -1,2 +1,2 @@\n alpha\n-beta\n+BETA\n")
    assert result == "alpha\nBETA"


def test_the_no_newline_marker_on_the_new_side_removes_the_trailing_newline():
    result = _apply(
        "alpha\nbeta\n",
        "@@ -1,2 +1,2 @@\n alpha\n-beta\n+BETA\n\\ No newline at end of file\n",
    )
    assert result == "alpha\nBETA"


def test_the_no_newline_marker_on_the_old_side_is_not_an_instruction():
    """git emits it for the old side too. Reading that as an instruction would
    strip a newline the patch never touched."""
    result = _apply(
        "alpha\nbeta\n",
        "@@ -1,2 +1,2 @@\n alpha\n-beta\n\\ No newline at end of file\n+BETA\n",
    )
    assert result == "alpha\nBETA\n"


# --- Refusing the unusable ---------------------------------------------------------
def test_a_patch_with_no_hunks_says_what_one_looks_like():
    with pytest.raises(patch.PatchError) as caught:
        patch.parse("just change line four please")
    assert "@@" in caught.value.message


def test_too_many_hunks_is_refused_and_points_at_file_write():
    diff = "".join(
        f"@@ -{n},1 +{n},1 @@\n-line {n}\n+LINE {n}\n" for n in range(1, patch.MAX_HUNKS + 5)
    )
    with pytest.raises(patch.PatchError) as caught:
        patch.parse(diff)
    assert "file_write" in caught.value.message


# --- Rendering -----------------------------------------------------------------------
def test_render_produces_a_diff_of_the_change():
    diff = patch.render("alpha\nbeta\n", "alpha\nBETA\n", "x.py")
    assert "-beta" in diff
    assert "+BETA" in diff
    assert "a/x.py" in diff


def test_render_is_bounded():
    """It goes on the message row forever and is re-parsed on every page load,
    and a generated file's diff can be larger than the file."""
    before = "\n".join(str(n) for n in range(500))
    after = "\n".join(f"x{n}" for n in range(500))
    diff = patch.render(before, after, "big.txt", max_lines=20)

    assert len(diff.split("\n")) <= 21
    assert "more lines" in diff