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

	// 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 {
	Node(level, index int) (Node, bool)
}

// 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 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 Taelin's rollback push list, this order makes the same merges
// as the push. The oldest pair wins a tie.
type View struct {
	high    int
	low     int
	lines   []nodeKey
	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,
	}
}

// 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 := tree.Node(line.level, line.index)
		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
}

// 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.
func (v *View) Append(tree nodeReader) {
	v.lines = append(v.lines, nodeKey{0, v.end})
	v.end++
	v.fit(tree)
}

func (v *View) fit(tree nodeReader) {
	size := v.Size(tree)
	if size > v.high {
		v.merging = true
	}

	for v.merging && size > v.low {
		best := v.mostDue(tree)
		if best < 0 {
			return
		}

		left, right := v.lines[best], v.lines[best+1]
		parent := nodeKey{left.level + 1, left.index / 2}
		size += lineSize(tree, parent) - lineSize(tree, left) - lineSize(tree, right)
		v.lines[best] = parent
		v.lines = append(v.lines[:best+1], v.lines[best+2:]...)
	}

	v.merging = false
}

// 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 {
	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
		}

		_, built := tree.Node(left.level+1, left.index/2)
		if !built {
			continue
		}

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

	return best
}

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

	return 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(tree nodeReader) string {
	var text strings.Builder
	text.WriteString("<chat>\n")
	for _, line := range v.lines {
		renderLine(&text, tree, line)
	}

	text.WriteString("</chat>")

	return text.String()
}

// lineSize returns the size of a view line.
func lineSize(tree nodeReader, line nodeKey) int {
	node, built := tree.Node(line.level, line.index)
	if !built {
		return len(pending)
	}

	return node.Size
}

// renderLine writes one view line, "id+n|text", and a newline.
func renderLine(text *strings.Builder, tree nodeReader, line nodeKey) {
	body := pending
	node, built := tree.Node(line.level, line.index)
	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')
}