#!/usr/bin/env python3
"""Repo location and toolchain discovery — the root fix for CB-WP-0004 T01.

CB-RES-0003 measured 84 turns and $15.33 spent on `cd` and
`export PATH="$HOME/.cargo/bin:$PATH"`. That friction had two causes:

1. tools resolved their inputs relative to the *caller's* working
   directory, so every invocation had to be preceded by a `cd`;
2. `cargo` is not on the default PATH, so every Rust-touching command had
   to be preceded by an `export`.

Both are fixed here, once, rather than at each leaf. `tools/dep-weight.py`
previously carried its own "cargo not on PATH" message — evidence the
friction was noticed and patched in the wrong place.

Single source of fact (InnerLoop v1.2): ROOT is derived here and imported;
it is not recomputed per tool.
"""
import os
import shutil

# tools/repo.py -> tools -> repo root.
ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))

# Where a rustup install puts the toolchain when PATH has not been set up.
CARGO_FALLBACKS = (
    os.path.expanduser("~/.cargo/bin"),
    "/usr/local/cargo/bin",
)


def enter_root():
    """Run from the repo root regardless of where the caller invoked us.

    Every tool calls this first. After it, plain relative paths
    (`specs/...`, `scenarios/...`) are correct, which is why the tools
    below did not need rewriting path-by-path.
    """
    os.chdir(ROOT)
    return ROOT


def cargo_bin():
    """Absolute path to cargo, or None.

    Checks PATH first so an explicitly-configured toolchain wins, then the
    standard rustup locations. Returning None rather than exiting keeps the
    policy (abort? skip?) with the caller.
    """
    found = shutil.which("cargo")
    if found:
        return found
    for d in CARGO_FALLBACKS:
        cand = os.path.join(d, "cargo")
        if os.path.isfile(cand) and os.access(cand, os.X_OK):
            return cand
    return None


def cargo_env():
    """Environment with the cargo bin dir prepended to PATH.

    For subprocesses that shell out to cargo indirectly. Returns a copy;
    never mutates os.environ.
    """
    env = dict(os.environ)
    cargo = cargo_bin()
    if cargo:
        env["PATH"] = os.path.dirname(cargo) + os.pathsep + env.get("PATH", "")
    return env


def self_test():
    """Positive control: this module must actually locate things.

    A resolver that silently returns a wrong-but-plausible answer is the
    harness-does-nothing class applied to paths — every downstream tool
    would then measure the wrong tree while reporting success.
    """
    results = []

    def check(name, ok, detail=""):
        results.append((name, ok, detail))

    check("ROOT is a directory", os.path.isdir(ROOT), ROOT)
    # Identified by content, not by name: a ROOT that pointed at the parent
    # directory would still be "a directory".
    check("ROOT is *this* repo",
          os.path.isfile(os.path.join(ROOT, "Cargo.toml"))
          and os.path.isdir(os.path.join(ROOT, "specs"))
          and os.path.isfile(os.path.join(ROOT, "INTENT.md")))
    # ROOT must not depend on the caller's cwd — the defect this fixes.
    here = os.getcwd()
    try:
        os.chdir("/")
        again = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
        check("ROOT is independent of the caller's cwd", again == ROOT)
    finally:
        os.chdir(here)

    cargo = cargo_bin()
    check("cargo is locatable without PATH setup", bool(cargo), cargo or "NOT FOUND")
    check("cargo_env puts cargo on PATH",
          not cargo or os.path.dirname(cargo) in cargo_env()["PATH"].split(os.pathsep))

    print("repo self-test (positive control)")
    ok = True
    for name, passed, detail in results:
        print(f"  [{'ok ' if passed else 'FAIL'}] {name}"
              + (f"  — {detail}" if detail else ""))
        ok &= passed
    return 0 if ok else 1


if __name__ == "__main__":
    import sys

    # This module has nothing to report but its own controls, so bare
    # invocation and --self-test are the same run. The flag exists because
    # `make self-tests` and loop-lint both look for it by name.
    if len(sys.argv) > 1 and sys.argv[1] not in ("--self-test",):
        print(f"usage: {sys.argv[0]} [--self-test]", file=sys.stderr)
        sys.exit(2)
    sys.exit(self_test())