package atomize

import (
	"context"
	"fmt"
	"math"
	"net/http"
	"strings"
	"time"
	"unicode/utf8"

	"bountyboard/internal/extsvc"
)

// Backend for the Anypreta "Issue Atomizer" API (OpenAPI 3.1, v0.1.0). Its
// model is two-level — issue → features → tasks — where ours is one-level
// (task → children), and only *features* carry an effort weight
// (`estimation`); its tasks carry none at all. A board child therefore maps to
// a Feature and Subdivide calls POST /v1/atomize-issue.
//
// Extend and Summarize have no counterpart here (/v1/extend-feature-tasks adds
// tasks *inside* one feature and returns the whole set), so those stay on the
// §5.1 service. The API declares no security scheme; it authenticates with
// Authorization: Bearer <key>.

// Field caps from the spec — exceeded values are 422s, so trim rather than fail.
const (
	maxIssueLen    = 50000
	maxGuidanceLen = 10000
	maxSystemIDLen = 256
	maxSystemDesc  = 2000
)

type apComponent struct {
	ID            string        `json:"id"`
	Description   string        `json:"description"`
	SubComponents []apComponent `json:"sub_components,omitempty"`
}

type apSystem struct {
	ID          string        `json:"id"`
	Description string        `json:"description"`
	Components  []apComponent `json:"components,omitempty"`
}

type apAtomizeIssueRequest struct {
	System       apSystem `json:"system"`
	Issue        string   `json:"issue"`
	GuidanceNote string   `json:"guidance_note,omitempty"`
}

type apCriterion struct {
	Description string `json:"description"`
}

type apFeature struct {
	ID                 string        `json:"id"`
	Title              string        `json:"title"`
	StakeholderSummary string        `json:"stakeholder_summary"`
	Description        string        `json:"description"`
	AcceptanceCriteria []apCriterion `json:"acceptance_criteria"`
	Priority           int           `json:"priority"`
	RelatedComponents  []string      `json:"related_components"`
	DependsOn          []string      `json:"depends_on"`
	// Estimation is optional and nullable in the spec; observed to sum to 1
	// across a feature set, but that is not promised.
	Estimation *float64 `json:"estimation"`
}

type apAtomizeIssueResponse struct {
	FeatureSet struct {
		Features []apFeature `json:"features"`
	} `json:"feature_set"`
}

// remote is the Anypreta-backed Atomize path. It reuses extsvc for the bearer
// auth, retry and circuit breaker; the base URL carries the /v1 prefix so
// extsvc's /healthz probe lands on the API's /v1/healthz.
type remote struct {
	c *extsvc.Client
}

func newRemote(baseURL, apiKey string, timeout time.Duration) *remote {
	base := strings.TrimRight(baseURL, "/") + "/v1"
	return &remote{c: extsvc.NewClient("atomizer-remote",
		func() string { return base }, apiKey, timeout)}
}

// Atomize maps a subdivide request onto POST /v1/atomize-issue and maps the
// returned features back onto board children.
func (r *remote) Atomize(ctx context.Context, req AtomizeRequest) (*AtomizeResponse, error) {
	body := apAtomizeIssueRequest{
		System:       systemFor(req),
		Issue:        truncate(issueText(req), maxIssueLen),
		GuidanceNote: truncate(guidanceNote(req), maxGuidanceLen),
	}
	var resp apAtomizeIssueResponse
	if err := r.c.Call(ctx, http.MethodPost, "/atomize-issue", body, &resp); err != nil {
		return nil, err
	}
	features := resp.FeatureSet.Features
	if len(features) == 0 {
		return nil, fmt.Errorf("%w: empty feature set", ErrBadCoefficients)
	}

	coeffs, estimated, err := featureCoefficients(features)
	if err != nil {
		return nil, err
	}
	tasks := make([]SubTask, len(features))
	for i, f := range features {
		criteria := make([]string, 0, len(f.AcceptanceCriteria))
		for _, c := range f.AcceptanceCriteria {
			if d := strings.TrimSpace(c.Description); d != "" {
				criteria = append(criteria, d)
			}
		}
		tasks[i] = SubTask{
			Title:              strings.TrimSpace(f.Title),
			Description:        strings.TrimSpace(f.Description),
			AcceptanceCriteria: criteria,
			EffortCoefficient:  coeffs[i],
		}
	}
	out := &AtomizeResponse{Tasks: tasks, Model: "anypreta/issue-atomizer"}
	if !estimated {
		out.Notes = "The atomizer returned no effort estimation, so the budget was split evenly across the tasks."
	}
	return out, nil
}

// featureCoefficients turns feature estimations into effort coefficients
// summing to exactly 1. Estimations are optional in the spec, so an incomplete
// set falls back to an even split (reported via the second return value)
// rather than failing the job — bounties still need *some* weight.
func featureCoefficients(features []apFeature) ([]float64, bool, error) {
	raw := make([]float64, len(features))
	estimated := true
	for i, f := range features {
		if f.Estimation == nil || *f.Estimation <= 0 || math.IsNaN(*f.Estimation) {
			estimated = false
			break
		}
		raw[i] = *f.Estimation
	}
	if !estimated {
		even := 1.0 / float64(len(features))
		for i := range raw {
			raw[i] = even
		}
	}
	norm, err := normalizeToOne(raw)
	if err != nil {
		return nil, false, err
	}
	return norm, estimated, nil
}

// normalizeToOne rescales positive weights proportionally so they sum to
// exactly 1. Unlike NormalizeCoefficients (§5.1) it accepts any positive sum:
// this API never promises a normalized feature set, so a deviation is expected
// rather than a service fault.
func normalizeToOne(weights []float64) ([]float64, error) {
	if len(weights) == 0 {
		return nil, fmt.Errorf("%w: empty", ErrBadCoefficients)
	}
	sum := 0.0
	for _, w := range weights {
		if math.IsNaN(w) || math.IsInf(w, 0) || w <= 0 {
			return nil, fmt.Errorf("%w: weight %v is not positive", ErrBadCoefficients, w)
		}
		sum += w
	}
	out := make([]float64, len(weights))
	largest, total := 0, 0.0
	for i, w := range weights {
		out[i] = math.Round(w/sum*10000) / 10000
		total += out[i]
		if out[i] > out[largest] {
			largest = i
		}
	}
	out[largest] = math.Round((out[largest]+1-total)*10000) / 10000
	return out, nil
}

// defaultComponents is the component tree we send for every customer.
//
// The spec marks `components` optional, but the service only emits features for
// components it considers *in scope*: an empty tree yields zero features and a
// 422 ("no in-scope components produced features"). The board has no component
// model of its own, so we send a generic tree covering the usual surfaces of a
// software change. Components irrelevant to a ticket simply produce nothing, so
// a broad tree costs nothing and a narrow one under-splits — a single catch-all
// component collapses every ticket into one feature.
var defaultComponents = []apComponent{
	{ID: "frontend", Description: "User interface: pages, views, client-side behaviour and styling."},
	{ID: "backend", Description: "Server-side application logic, APIs and background jobs."},
	{ID: "data", Description: "Data storage: schema, migrations, queries and data integrity."},
	{ID: "integrations", Description: "Integrations with external systems, third-party APIs and notifications."},
}

// systemFor derives the required system object from the customer, which is the
// only project-level context the board has.
func systemFor(req AtomizeRequest) apSystem {
	id := strings.TrimSpace(req.CustomerID)
	if id == "" {
		id = "bountyboard"
	}
	name := strings.TrimSpace(req.CustomerName)
	if name == "" {
		name = "this project"
	}
	return apSystem{
		ID:          truncate(id, maxSystemIDLen),
		Description: truncate(fmt.Sprintf("Software project %q. Work is delivered as small, independently deliverable tasks picked up by freelance developers.", name), maxSystemDesc),
		Components:  defaultComponents,
	}
}

// issueText flattens the ticket into the single `issue` string the API takes —
// it has no fields for acceptance criteria, links or attachments.
func issueText(req AtomizeRequest) string {
	var b strings.Builder
	if t := strings.TrimSpace(req.Title); t != "" {
		fmt.Fprintf(&b, "%s\n\n", t)
	}
	if d := strings.TrimSpace(req.Description); d != "" {
		fmt.Fprintf(&b, "%s\n", d)
	}
	if len(req.AcceptanceCriteria) > 0 {
		b.WriteString("\nAcceptance criteria:\n")
		for _, c := range req.AcceptanceCriteria {
			fmt.Fprintf(&b, "- %s\n", c)
		}
	}
	if len(req.Links) > 0 {
		b.WriteString("\nLinks:\n")
		for _, l := range req.Links {
			fmt.Fprintf(&b, "- %s\n", l)
		}
	}
	if len(req.Attachments) > 0 {
		b.WriteString("\nAttachments:\n")
		for _, a := range req.Attachments {
			fmt.Fprintf(&b, "- %s (%s): %s\n", a.Name, a.MimeType, a.URL)
		}
	}
	s := strings.TrimSpace(b.String())
	if s == "" {
		s = "(no description provided)"
	}
	return s
}

// guidanceNote carries the consultant's note plus our task-count constraints,
// which the API has no dedicated field for.
func guidanceNote(req AtomizeRequest) string {
	parts := make([]string, 0, 2)
	if n := strings.TrimSpace(req.SubdivisionNote); n != "" {
		parts = append(parts, n)
	}
	if c := req.Constraints; c != nil {
		switch {
		case c.MinTasks > 0 && c.MaxTasks > 0:
			parts = append(parts, fmt.Sprintf("Produce between %d and %d features.", c.MinTasks, c.MaxTasks))
		case c.MinTasks > 0:
			parts = append(parts, fmt.Sprintf("Produce at least %d features.", c.MinTasks))
		case c.MaxTasks > 0:
			parts = append(parts, fmt.Sprintf("Produce at most %d features.", c.MaxTasks))
		}
	}
	return strings.Join(parts, "\n\n")
}

// truncate caps a string at max bytes without splitting a rune. Byte length is
// a safe proxy for the spec's character limits: it is never smaller.
func truncate(s string, max int) string {
	if len(s) <= max {
		return s
	}
	cut := max
	for cut > 0 && !utf8.RuneStart(s[cut]) {
		cut--
	}
	return s[:cut]
}