package main

import (
	"fmt"

	"github.com/git-bug/git-bug/entities/bug"
)

// parentMetadataKey is the CreateOp metadata key holding the full ID
// of the parent bug. Absent means "no parent".
//
// git-bug metadata keys are immutable once set: a later SetMetadata
// cannot overwrite or delete the key. A parent link is added in place to
// a bug that has none; changing an existing link re-creates the bug (see
// applyCreateMetadata). Links cannot be removed.
const parentMetadataKey = "parent"

// currentParentID returns the bug's current parent full ID, or "".
func currentParentID(snap *bug.Snapshot) string {
	if parent, ok := snap.GetCreateMetadata(parentMetadataKey); ok && parent != "" {
		return parent
	}
	return ""
}

// createsParentCycle reports whether linking bugFullID under parentFullID
// would close a dependency cycle (i.e. bugFullID is already an ancestor
// of parentFullID). Self-links are also reported as cycles.
func createsParentCycle(issues []BugIssue, bugFullID, parentFullID string) bool {
	if bugFullID == parentFullID {
		return true
	}
	byID := make(map[string]BugIssue, len(issues))
	for _, issue := range issues {
		byID[issue.FullID] = issue
	}
	for current := parentFullID; current != ""; {
		if current == bugFullID {
			return true
		}
		ancestor, ok := byID[current]
		if !ok {
			return false
		}
		current = ancestor.ParentID
	}
	return false
}

// resolveNewParent validates a requested parent for bug b and returns the
// parent's full ID together with all bugs (for cycle and subtree checks).
// An empty parentID returns "" and no bugs.
func resolveNewParent(repoPath string, b *bug.Bug, parentID string) (string, []BugIssue, error) {
	if parentID == "" {
		return "", nil, nil
	}
	parentBug, err := resolveBugID(repoPath, parentID)
	if err != nil {
		return "", nil, fmt.Errorf("failed to resolve parent bug ID %q: %w\n\nUse 'bug ls' to see available bugs", parentID, err)
	}
	parentFullID := parentBug.Id().String()
	bugFullID := b.Id().String()
	if parentFullID == bugFullID {
		return "", nil, fmt.Errorf("bug cannot be its own parent")
	}
	if replacement := supersededBy(b.Compile()); replacement != "" {
		return "", nil, fmt.Errorf("bug %s was superseded by %s; edit %s instead", shortID(bugFullID), shortID(replacement), shortID(replacement))
	}
	if replacement := supersededBy(parentBug.Compile()); replacement != "" {
		return "", nil, fmt.Errorf("parent %s was superseded by %s; use %s as the parent instead", shortID(parentFullID), shortID(replacement), shortID(replacement))
	}
	issues, err := LoadBugs(repoPath)
	if err != nil {
		return "", nil, fmt.Errorf("failed to load bugs: %w", err)
	}
	if createsParentCycle(issues, bugFullID, parentFullID) {
		return "", nil, fmt.Errorf("setting parent %q would create a dependency cycle", parentID)
	}
	return parentFullID, issues, nil
}