package main

import (
	"context"
	"errors"
	"fmt"
	"slices"
	"strings"
	"unicode/utf8"
)

const (
	// compactionTries is the most calls for one node.
	compactionTries = 5

	// viewBlock is the number of view lines in one input part.
	viewBlock = 4
)

// ruler shows the model the size limit of a line. Models cannot count bytes,
// and a sample line as the ruler gets its content copied.
var ruler = strings.Repeat("-", nodeBytes)

// Compactor builds the nodes of one agent with a model. A compaction is a call
// like a turn, with the same system prompt and tools, so it reads them from the
// prompt cache of the turns. The tools are never called. After them comes one
// user message with the compaction view up to the node and the task, in the
// parts that compactionParts gives. Only the last part holds the end of the
// view, so compactions whose views end at different lines begin with the same
// parts up to their last shared block.
//
// When the line is over nodeBytes, the next try sends the same view and task,
// plus a note with the size of the newest line and its cut at the limit. A try
// never sends the model's earlier reply or reasoning back, so every try has the
// same cached prefix and about the same size. After at most compactionTries
// tries, the shortest line is the node. A few bytes over the limit is fine,
// because the view measures the real sizes. A call that fails before any line
// fails the build, and the core tries it again at the next message.
type Compactor struct {
	client       *Client
	model        ModelConfig
	instructions string
	tools        []Tool
}

// newCompactor returns the compactor of the agent name, which sends its calls
// through client with model.
func newCompactor(client *Client, name string, model ModelConfig) *Compactor {
	return &Compactor{
		client:       client,
		model:        model,
		instructions: prompt(name),
		tools:        agentTools(),
	}
}

// Summarize returns the line of the node of task.
func (c *Compactor) Summarize(ctx context.Context, task Task) (string, error) {
	text, err := compactionTask(task)
	if err != nil {
		return "", err
	}

	first := compactionParts(task.Context, text)
	var lines []string
	note := ""
	for len(lines) < compactionTries {
		parts := first
		if note != "" {
			parts = append(slices.Clip(first), note)
		}

		var response Response
		response, err = c.client.Create(ctx, Request{
			Model:        c.model.Model,
			Effort:       c.model.Effort,
			Instructions: c.instructions,
			Tools:        c.tools,
			Input:        []any{userMessage(parts...)},
			Node:         &nodeKey{task.Level, task.Index},
		})
		if err != nil {
			break
		}

		line := strings.TrimSpace(response.Text)
		if line == "" {
			err = errors.New("compaction gave no line")
			break
		}

		lines = append(lines, line)
		if len(line) <= nodeBytes {
			break
		}

		note = tooLong(line)
	}

	if len(lines) == 0 {
		return "", fmt.Errorf("compact node (%d, %d): %w", task.Level, task.Index, err)
	}

	return shortest(lines), nil
}

// compactionParts returns the input parts of a first try for the compaction
// view context and the task text: the opening tag, one part for each whole
// block of viewBlock lines, and then one part with the rest of the lines, the
// closing tag, a newline, and the task. Joined, the parts are the view, a
// newline, and the task.
func compactionParts(context, task string) []string {
	lines := strings.SplitAfter(strings.TrimPrefix(context, "<chat>\n"), "\n")
	whole := len(lines) - 1 - (len(lines)-1)%viewBlock
	parts := []string{"<chat>\n"}
	for n := 0; n < whole; n += viewBlock {
		parts = append(parts, strings.Join(lines[n:n+viewBlock], ""))
	}

	return append(parts, strings.Join(lines[whole:], "")+"\n"+task)
}

// compactionTask returns the task of a build, with its input.
func compactionTask(task Task) (string, error) {
	if task.Level == 0 && len(task.Input) == 1 {
		return fmt.Sprintf(`Compaction: compress message %d into one line of at most %d bytes
(about 70 words), the length of this ruler:
%s
<input>
%s
</input>`, task.Index, nodeBytes, ruler, task.Input[0]), nil
	}

	if task.Level > 0 && len(task.Input) == 2 {
		first := task.Index << task.Level
		half := 1 << (task.Level - 1)

		return fmt.Sprintf(`Compaction: merge lines %d+%d and %d+%d, adjacent, into one line of at most
%d bytes (about 70 words), the length of this ruler:
%s
<chat> may hold their messages, %d to %d, in more detail: take details
of them from there too.
<input>
%s
%s
</input>`, first, half, first+half, half, nodeBytes, ruler, first, nodeEnd(task.Level, task.Index)-1, task.Input[0], task.Input[1]), nil
	}

	return "", fmt.Errorf("node (%d, %d) has %d inputs", task.Level, task.Index, len(task.Input))
}

// tooLong returns the note for a line over nodeBytes: its size and its first
// nodeBytes bytes, which show the model where the limit cuts it.
func tooLong(line string) string {
	return fmt.Sprintf(`Too long: your last line for this <input> was %d bytes,
over the %d-byte limit. Write the whole line again, cutting just
enough of the least valuable items to fit before this cut:
%s| <- LIMIT`, len(line), nodeBytes, cutLine(line))
}

// cutLine returns the first nodeBytes bytes of line, without a part of a
// character at the end.
func cutLine(line string) string {
	if len(line) <= nodeBytes {
		return line
	}

	cut := line[:nodeBytes]
	for !utf8.ValidString(cut) {
		cut = cut[:len(cut)-1]
	}

	return cut
}

// shortest returns the first of the shortest lines.
func shortest(lines []string) string {
	best := lines[0]
	for _, line := range lines[1:] {
		if len(line) < len(best) {
			best = line
		}
	}

	return best
}