fejkdata/reference.go

package fejkdata

import (
	"fmt"
	"strings"
)

// A reference names a node by path rather than as a sibling field: {/a.b} from the
// data root, {.a} from the folder this file sits in, {..a} from the folder above.
func isRef(name string) bool { return strings.HasPrefix(name, ".") || strings.HasPrefix(name, "/") }

// refBinding is what a reference was bound to: the head key its category is held
// under, and the tail read into it.
type refBinding struct {
	key  string
	tail []string
}

// refShape splits a reference into its sigil and the dotted path after it.
func refShape(name string) (sigil, rest string, err error) {
	switch {
	case strings.HasPrefix(name, "/"):
		sigil, rest = "/", name[1:]
	case strings.HasPrefix(name, ".."):
		sigil, rest = "..", name[2:]
	default:
		sigil, rest = ".", name[1:]
	}
	if rest == "" {
		return "", "", fmt.Errorf("reference has no path")
	}
	if strings.HasPrefix(rest, "/") {
		return "", "", fmt.Errorf("the path after %s starts at a name, not a /; write {%s%s}", sigil, sigil, rest[1:])
	}
	if strings.HasPrefix(rest, ".") {
		return "", "", fmt.Errorf("a reference starts with / (the root), . (this folder) or .. (the folder above)")
	}
	for _, seg := range strings.Split(rest, ".") {
		if seg == "" {
			return "", "", fmt.Errorf("path has an empty segment")
		}
	}
	return sigil, rest, nil
}

// refSegments resolves a reference written in folder to a path from the root.
func refSegments(name string, folder []string) ([]string, error) {
	sigil, rest, err := refShape(name)
	if err != nil {
		return nil, err
	}
	var base []string
	switch sigil {
	case ".":
		base = folder
	case "..":
		if len(folder) == 0 {
			return nil, fmt.Errorf("no folder above the root")
		}
		base = folder[:len(folder)-1]
	}
	return append(append([]string{}, base...), strings.Split(rest, ".")...), nil
}

// linkRefs resolves every reference in the assembled tree. The head of the path —
// up to the category it names — is bound into the referring template's fields
// under its root path, and the rest reads into it the way a sibling path does, so
// a reference is held like a sibling and two spellings of one target are one
// draw. It runs once, after all data is merged, so a reference sees the final
// (override-resolved) tree. A path that is unknown, names a folder, or reads a
// field not every variant carries fails here, keeping a bad reference a New-time
// error, never a random render-time one.
func linkRefs(root map[string]node) error {
	return eachTemplate(root, func(folder []string, path string, t *template) error {
		names := refTokens(t.format)
		if len(names) == 0 {
			return nil
		}
		if t.fields == nil {
			t.fields = map[string]node{}
		}
		t.refs = make(map[string]refBinding, len(names))
		for _, name := range names {
			segments, err := refSegments(name, folder)
			if err != nil {
				return fmt.Errorf("%s: reference {%s}: %w", path, name, err)
			}
			head, target, tail, err := resolveRef(root, segments)
			if err != nil {
				return fmt.Errorf("%s: reference {%s}: %w", path, name, err)
			}
			key := "/" + strings.Join(head, ".")
			if err := checkPath(target, tail, key); err != nil {
				return fmt.Errorf("%s: reference {%s}: %w", path, name, err)
			}
			t.fields[key] = target
			t.refs[name] = refBinding{key, tail}
		}
		if err := t.compileFormat(); err != nil {
			return fmt.Errorf("%s: %w", path, err)
		}
		return nil
	})
}

// eachTemplate calls fn once per template, with the folder its category sits in
// and the dot path reaching it, folders and names in sorted order.
func eachTemplate(root map[string]node, fn func(folder []string, path string, t *template) error) error {
	var inCategory func(folder []string, path string, n node) error
	inCategory = func(folder []string, path string, n node) error {
		if t, ok := n.(*template); ok {
			if err := fn(folder, path, t); err != nil {
				return err
			}
		}
		for _, c := range contained(n) {
			if err := inCategory(folder, join(path, c.name), c.node); err != nil {
				return err
			}
		}
		return nil
	}
	var inFolder func(folder []string, children map[string]node) error
	inFolder = func(folder []string, children map[string]node) error {
		for _, name := range sortedNames(children) {
			path := join(strings.Join(folder, "."), name)
			if g, ok := children[name].(*group); ok {
				if err := inFolder(append(folder[:len(folder):len(folder)], name), g.children); err != nil {
					return err
				}
				continue
			}
			if err := inCategory(folder, path, children[name]); err != nil {
				return err
			}
		}
		return nil
	}
	return inFolder(nil, root)
}

// resolveRef walks a reference path through the folders to the category it names,
// returning that head, the node, and the tail left to read into it. A descent of
// its own rather than a walkPath: it walks groups only and returns where they end,
// not a leaf.
func resolveRef(root map[string]node, segments []string) (head []string, target node, tail []string, err error) {
	var n node = &group{children: root}
	i := 0
	for ; i < len(segments); i++ {
		g, ok := n.(*group)
		if !ok {
			break
		}
		child, ok := g.children[segments[i]]
		if !ok {
			return nil, nil, nil, fmt.Errorf("no entry %q", segments[i])
		}
		n = child
	}
	if _, ok := n.(*group); ok {
		return nil, nil, nil, fmt.Errorf("names a folder, not a value")
	}
	return segments[:i], n, segments[i:], nil
}

// refTokens returns the reference names a format reads, as tokens or as operands.
func refTokens(format string) []string {
	var refs []string
	seen := map[string]bool{}
	for _, name := range append(fieldTokens(format), operandTokens(format)...) {
		if isRef(name) && !seen[name] {
			seen[name] = true
			refs = append(refs, name)
		}
	}
	return refs
}