package fakes

import (
	"fmt"
	"sort"
	"strings"
)

// rng is the randomness the renderer draws from. Passing it in keeps the render
// functions a pure core over an explicit effect; *rand.Rand satisfies it.
type rng interface {
	IntN(n int) int
	Float64() float64
}

// Fake generates a value for a dot path. Each segment descends one level: folder
// names and the category (JSON file) come first, then named fields within it,
// e.g. "sv_SE.address" or "sv_SE.address.street". Choices along the way are
// resolved at random. A path naming a folder (no value of its own) is an error.
func (f *Fakes) Fake(path string) (string, error) {
	n, err := descend(f.rand, &group{children: f.categories}, strings.Split(path, "."))
	if err != nil {
		return "", fmt.Errorf("fakes: %s: %w", path, err)
	}
	if _, ok := n.(*group); ok {
		return "", fmt.Errorf("fakes: %s names a folder, not a value", path)
	}
	return render(f.rand, n), nil
}

// descend walks named fields to the node a path names. It is the one render-side
// step that can fail, because the path comes from the caller and may name a field
// that does not exist. A choice consumes no segment, so the rest of the path must
// be one every variant carries (the set compile stored) before a variant is picked
// — a path that resolves at all resolves on every call.
func descend(s *session, n node, segments []string) (node, error) {
	if len(segments) == 0 {
		return n, nil
	}
	switch n := n.(type) {
	case *group:
		child, ok := n.children[segments[0]]
		if !ok {
			return nil, fmt.Errorf("no entry %q", segments[0])
		}
		return descend(s, child, segments[1:])
	case *template:
		child, ok := n.fields[segments[0]]
		if !ok {
			return nil, fmt.Errorf("no field %q", segments[0])
		}
		return descend(s, child, segments[1:])
	case *choice:
		if len(n.items) > 1 {
			if want := strings.Join(segments, "."); !n.shared[want] {
				return nil, unreachableInChoice(n, want)
			}
		}
		return descend(s, pick(s, n), segments)
	case literal:
		return nil, fmt.Errorf("a plain string has no field %q", segments[0])
	default:
		return nil, fmt.Errorf("cannot descend into %T at %q", n, segments[0])
	}
}

// unreachableInChoice reports that a path cannot step through this choice, listing
// what every variant does carry. It reads the precomputed set, so a failing path
// costs no more than a rendering one.
func unreachableInChoice(c *choice, want string) error {
	if len(c.shared) == 0 {
		return fmt.Errorf("no variant of this %d-way choice carries %q", len(c.items), want)
	}
	offered := make([]string, 0, len(c.shared))
	for p := range c.shared {
		offered = append(offered, p)
	}
	sort.Strings(offered)
	return fmt.Errorf("not every variant of this %d-way choice carries %q; all carry %v", len(c.items), want, offered)
}

// render evaluates a compiled node to a string. compile validates every node up
// front, so rendering a compiled tree cannot fail.
func render(s *session, n node) string {
	switch n := n.(type) {
	case literal:
		return string(n)
	case *choice:
		return render(s, pick(s, n))
	case *template:
		if n.repeat == 1 {
			return expand(s, n)
		}
		var b strings.Builder
		b.Grow(n.repeat * (n.grow + len(n.separator)))
		for i := 0; i < n.repeat; i++ {
			if i > 0 {
				b.WriteString(n.separator)
			}
			b.WriteString(expand(s, n))
		}
		return b.String()
	default:
		panic(fmt.Sprintf("fakes: uncompiled node %T", n))
	}
}

// pick selects one item. Uniform choices are O(1); weighted choices are an
// O(log n) search over precomputed cumulative weights. compile guarantees a
// non-empty choice and a finite positive total, so the index is always in range.
func pick(r rng, c *choice) node {
	if c.cum == nil {
		return c.items[r.IntN(len(c.items))]
	}
	x := r.Float64() * c.cum[len(c.cum)-1]
	i := sort.Search(len(c.cum), func(i int) bool { return c.cum[i] > x })
	return c.items[i]
}

// classChar randomises one character-class rune: '0' digit 0-9, '1' digit 1-9,
// 'A' letter A-Z, 'a' letter a-z.
func classChar(s *session, c rune) byte {
	switch c {
	case '0':
		return byte('0' + s.IntN(10))
	case '1':
		return byte('1' + s.IntN(9))
	case 'A':
		return byte('A' + s.IntN(26))
	default: // 'a'
		return byte('a' + s.IntN(26))
	}
}

// expand renders a template's compiled ops. compile validated every token, so this
// cannot fail.
func expand(s *session, t *template) string {
	var b strings.Builder
	b.Grow(t.grow)
	// One draw per bound head, held for this expansion only: a nested template and
	// each repeat iteration get their own, since each is its own expansion.
	var bound *draws
	if len(t.bound) > 0 {
		bound = &draws{
			variant: make(map[string]node, len(t.bound)),
			value:   make(map[string]string, len(t.bound)),
		}
	}
	for i := range t.ops {
		o := &t.ops[i]
		switch o.kind {
		case 'l':
			b.WriteString(o.lit)
		case 'c':
			b.WriteByte(classChar(s, o.r))
		case 'f':
			b.WriteString(resolve(s, o.arms, t, bound))
		case 'b':
			b.WriteString(o.call(s, b.String(), t.fields)) // b.String() is the output so far
		}
	}
	return b.String()
}

// draws is what an expansion has already drawn for its bound heads: the variant
// each head was drawn as, so every path under it reads one row, and the value each
// path read, so the same path read twice reads one value.
type draws struct {
	variant map[string]node
	value   map[string]string
}

// resolve renders one field alternation: the '|' alternatives, one picked at
// random. An arm's key is a sibling field or a {..path} reference, which linkRefs
// bound into fields too. A field the format addresses by dotted path is bound: it
// is drawn once for the expansion, so {place.postal-code} and {place.locality}
// read one row and either read twice gives one value. checkTokens, checkPath and
// linkRefs prove every step, so this cannot fail.
func resolve(s *session, arms []arm, t *template, bound *draws) string {
	a := arms[s.IntN(len(arms))]
	if !t.bound[a.key] {
		return render(s, t.fields[a.key])
	}
	if v, read := bound.value[a.name]; read {
		return v
	}
	n, drew := bound.variant[a.key]
	if !drew {
		n = drawn(s, t.fields[a.key])
		bound.variant[a.key] = n
	}
	if len(a.tail) > 0 {
		var err error
		if n, err = descend(s, n, a.tail); err != nil {
			panic(fmt.Sprintf("fakes: %s: %v", a.name, err))
		}
	}
	v := render(s, n)
	bound.value[a.name] = v
	return v
}

// drawn resolves a choice to one variant, so a bound head is a concrete node the
// rest of the expansion shares. Nested choices unwrap too: a draw is one value, not
// another set to pick from.
func drawn(s *session, n node) node {
	for c, ok := n.(*choice); ok; c, ok = n.(*choice) {
		n = pick(s, c)
	}
	return n
}