package main

import (
	"fmt"
	"math"
	"math/bits"
	"strconv"
	"strings"
)

const (
	// chatHigh and chatLow are the limits of the chat view in bytes.
	chatHigh = 128000
	chatLow  = 64000

	// compactionHigh and compactionLow are the limits of the compaction
	// view in bytes.
	compactionHigh = 32000
	compactionLow  = 16000

	// pending is the text of a line that is not built yet.
	pending = "(not summarized yet: zoom it)"
)

// nodeReader gives the built nodes of a tree.
type nodeReader interface {
	Built(level, index int) (bool, error)
	Node(level, index int) (Node, bool, error)
}

// View is a list of tree nodes that covers messages 0 to end-1 without a gap,
// oldest first. Each new message appends its leaf as a line. Once the size of
// the view goes above high, one batch merges the most due sibling pairs until
// the size is at most low. A batch merges only pairs whose parent is built. If
// the batch cannot get to low, it continues at each new message until it does.
//
// The view keeps the nodes of its built lines in memory, so its size and its
// text need no read of the tree. The size of a line is the size of its node
// text, or the size of the pending text when the node is not built.
//
// For the sibling pair (l, i) and (l, i+1), due is (T - last) / 2^l, with T the
// number of messages and last the ID of the last message of the pair. That is
// how long ago the pair ended, measured in its own line size. With a budget of
// the length of the rollback push list of the reference design, this order
// makes the same merges as the push. The oldest pair wins a tie.
type View struct {
	high    int
	low     int
	lines   []nodeKey
	nodes   map[nodeKey]Node
	end     int
	merging bool
}

// viewState is the saved form of a View. The limits are not part of it.
type viewState struct {
	Lines   [][2]int `json:"lines"`
	Merging bool     `json:"merging"`
}

// newView returns an empty view with the limits high and low.
func newView(high, low int) *View {
	return &View{
		high:  high,
		low:   low,
		nodes: make(map[nodeKey]Node),
	}
}

// restoreView returns the view of state with the limits high and low. Each line
// must continue the one before it, and each line above level 0 must be built.
func restoreView(state viewState, high, low int, tree nodeReader) (*View, error) {
	v := newView(high, low)
	v.merging = state.Merging
	for _, pair := range state.Lines {
		line := nodeKey{pair[0], pair[1]}
		if line.level < 0 || line.level >= bits.UintSize-2 || line.index < 0 || line.index<<line.level != v.end {
			return nil, fmt.Errorf("view line (%d, %d) does not start at message %d", line.level, line.index, v.end)
		}

		built, err := v.load(tree, line)
		if err != nil {
			return nil, err
		}

		if line.level > 0 && !built {
			return nil, fmt.Errorf("view line (%d, %d) is not built", line.level, line.index)
		}

		v.lines = append(v.lines, line)
		v.end = nodeEnd(line.level, line.index)
	}

	return v, nil
}

// load keeps the node of line when it is built, and tells if it is.
func (v *View) load(tree nodeReader, line nodeKey) (bool, error) {
	node, built, err := tree.Node(line.level, line.index)
	if err != nil || !built {
		return false, err
	}

	v.nodes[line] = node

	return true, nil
}

// State returns the saved form of the view.
func (v *View) State() viewState {
	state := viewState{
		Lines:   make([][2]int, len(v.lines)),
		Merging: v.merging,
	}

	for n, line := range v.lines {
		state.Lines[n] = [2]int{line.level, line.index}
	}

	return state
}

// Append adds the leaf of the next message as a line and merges when the view
// is too large. It tells if a merge changed the view.
func (v *View) Append(tree nodeReader) (bool, error) {
	line := nodeKey{0, v.end}
	_, err := v.load(tree, line)
	if err != nil {
		return false, err
	}

	v.lines = append(v.lines, line)
	v.end++

	return v.fit(tree)
}

// fit merges the most due pairs while a batch runs, and tells if a merge
// changed the view.
func (v *View) fit(tree nodeReader) (bool, error) {
	size := v.Size()
	if size > v.high {
		v.merging = true
	}

	merged := false
	for v.merging && size > v.low {
		best, err := v.mostDue(tree)
		if err != nil || best < 0 {
			return merged, err
		}

		left, right := v.lines[best], v.lines[best+1]
		parent := nodeKey{left.level + 1, left.index / 2}
		_, err = v.load(tree, parent)
		if err != nil {
			return merged, err
		}

		size += v.lineSize(parent) - v.lineSize(left) - v.lineSize(right)
		delete(v.nodes, left)
		delete(v.nodes, right)
		v.lines[best] = parent
		v.lines = append(v.lines[:best+1], v.lines[best+2:]...)
		merged = true
	}

	v.merging = false

	return merged, nil
}

// Coarsen takes each merge of other, a view of the same messages, and then
// merges in one batch until the size is at most low. Each line of the result is
// the coarser of the two lines that start at its first message. Dyadic ranges
// either nest or do not overlap, so the result covers the messages without a
// gap.
func (v *View) Coarsen(other *View, tree nodeReader) error {
	lines := make([]nodeKey, 0, len(v.lines))
	nodes := make(map[nodeKey]Node, len(v.nodes))
	mine, theirs := 0, 0
	for start := 0; start < v.end; {
		line := v.lines[mine]
		node, built := v.nodes[line]
		if other.lines[theirs].level > line.level {
			line = other.lines[theirs]
			node, built = other.nodes[line]
		}

		lines = append(lines, line)
		if built {
			nodes[line] = node
		}

		start = nodeEnd(line.level, line.index)
		for mine < len(v.lines) && v.lines[mine].index<<v.lines[mine].level < start {
			mine++
		}

		for theirs < len(other.lines) && other.lines[theirs].index<<other.lines[theirs].level < start {
			theirs++
		}
	}

	v.lines = lines
	v.nodes = nodes
	v.merging = true
	_, err := v.fit(tree)

	return err
}

// mostDue returns the position of the left line of the most due sibling pair
// whose parent is built, or -1 when there is no such pair.
func (v *View) mostDue(tree nodeReader) (int, error) {
	best := -1
	bestDue := 0.0
	for n := 0; n+1 < len(v.lines); n++ {
		left, right := v.lines[n], v.lines[n+1]
		if left.index%2 != 0 || right.level != left.level || right.index != left.index+1 {
			continue
		}

		last := nodeEnd(right.level, right.index) - 1
		due := math.Ldexp(float64(v.end-last), -left.level)
		if best >= 0 && due <= bestDue {
			continue
		}

		built, err := tree.Built(left.level+1, left.index/2)
		if err != nil {
			return -1, err
		}

		if built {
			best = n
			bestDue = due
		}
	}

	return best, nil
}

// Refresh keeps node when it is the node of a pending line.
func (v *View) Refresh(node Node) {
	key := nodeKey{node.Level, node.Index}
	for n := len(v.lines) - 1; n >= 0; n-- {
		line := v.lines[n]
		if line == key {
			v.nodes[key] = node
			return
		}

		if line.index<<line.level <= key.index<<key.level {
			return
		}
	}
}

// Size returns the sum of the line sizes.
func (v *View) Size() int {
	size := 0
	for _, line := range v.lines {
		size += v.lineSize(line)
	}

	return size
}

// lineSize returns the size of a view line.
func (v *View) lineSize(line nodeKey) int {
	node, built := v.nodes[line]
	if !built {
		return len(pending)
	}

	return node.Size
}

// Render returns the view as the model reads it: one line "id+n|text" for each
// node inside <chat> tags, with the newlines of the text as spaces.
func (v *View) Render() string {
	return v.RenderWithout(nil)
}

// RenderWithout returns the view as Render does, without the leaf lines of the
// messages in skip. A line that merges such a message with others stays.
func (v *View) RenderWithout(skip map[int]bool) string {
	var text strings.Builder
	text.WriteString("<chat>\n")
	for _, line := range v.lines {
		if line.level == 0 && skip[line.index] {
			continue
		}

		v.renderLine(&text, line)
	}

	text.WriteString("</chat>")

	return text.String()
}

// renderLine writes one view line, "id+n|text", and a newline.
func (v *View) renderLine(text *strings.Builder, line nodeKey) {
	body := pending
	node, built := v.nodes[line]
	if built {
		body = strings.ReplaceAll(node.Text, "\n", " ")
	}

	text.WriteString(strconv.Itoa(line.index << line.level))
	text.WriteByte('+')
	text.WriteString(strconv.Itoa(1 << line.level))
	text.WriteByte('|')
	text.WriteString(body)
	text.WriteByte('\n')
}

// Context returns the lines of the view before message end as the model reads
// them. It stops at the first line that is not built, so a compaction never
// sees a pending mark or a part of a message.
func (v *View) Context(end int) string {
	var text strings.Builder
	text.WriteString("<chat>\n")
	for _, line := range v.lines {
		_, built := v.nodes[line]
		if !built || nodeEnd(line.level, line.index) > end {
			break
		}

		v.renderLine(&text, line)
	}

	text.WriteString("</chat>")

	return text.String()
}