// Package config loads the two hand-edited files the tool reads: the
// data-root-wide rules.toml, and one account.toml per account folder.
package config

import (
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"

	"github.com/BurntSushi/toml"
)

const (
	// RulesFile is the rules file at the root of the data directory.
	RulesFile = "rules.toml"
	// AccountFile is the per-account config inside each account folder.
	AccountFile = "account.toml"
	// IndexFile is the rebuildable SQLite index, at the root of the data
	// directory alongside rules.toml.
	IndexFile = "index.db"
)

// Rule is one entry in rules.toml. Rules are evaluated in file order and the
// first one whose Match (and optional Account) matches wins.
type Rule struct {
	Match   string `toml:"match"`
	Tag     string `toml:"tag"`
	Account string `toml:"account"` // optional: restrict to one account slug
	// Type matches the bank's own classification, e.g. Revolut's CARD_PAYMENT.
	// Optional; when set, it must match as well as Match.
	Type string `toml:"type"`
	// Note is free text for the reader: why the rule exists, or what the
	// unrecognisable payee behind the glob actually is. It never affects
	// matching. It is a key rather than a comment so it survives a round trip
	// through LoadRules and can be shown on the rules screen.
	Note string `toml:"note"`
}

// Transfer is one entry in rules.toml describing money moved between two
// accounts the user owns. Both sides are named: a transfer is only ever a pair,
// which is what lets the report drop it without leaving half a movement behind.
//
// All four patterns are required. A one-sided definition would be a rule.
type Transfer struct {
	FromAccount string `toml:"from_account"`
	FromDesc    string `toml:"from_desc"` // glob vs. the leaving leg's description
	ToAccount   string `toml:"to_account"`
	ToDesc      string `toml:"to_desc"` // glob vs. the arriving leg's description
	// TolerancePct is how far short (or over) the arriving leg may be and still
	// count as the same movement, as a percentage of the leg that left. It exists
	// for routes where the bank takes a fee on the way, so the two statements
	// genuinely disagree about the amount.
	//
	// Zero — the default, and what every definition written before this key
	// existed means — requires the exact opposite amount. Keep it that way unless
	// a route actually charges: the wider the tolerance, the more likely two
	// unrelated movements in one window pair with each other.
	//
	// It applies within one currency only. Across currencies the amount is not
	// checked at all, so there is nothing for a tolerance to loosen.
	TolerancePct float64 `toml:"tolerance_pct"`
	// Note is free text for the reader, exactly as on a Rule: it never takes
	// part in matching.
	Note string `toml:"note"`
}

// Rules is the parsed rules.toml: the tagging rules and the transfer
// definitions, which share the file because they are both hand-maintained
// statements about the same transactions.
type Rules struct {
	Rule     []Rule     `toml:"rule"`
	Transfer []Transfer `toml:"transfer"`
}

// LoadRules reads rules.toml from the data root. A missing file is not an
// error: it just means nothing is tagged automatically yet.
func LoadRules(root string) (*Rules, error) {
	path := filepath.Join(root, RulesFile)
	var r Rules
	if _, err := toml.DecodeFile(path, &r); err != nil {
		if os.IsNotExist(err) {
			return &r, nil
		}
		return nil, fmt.Errorf("%s: %w", path, err)
	}
	for i, rule := range r.Rule {
		if rule.Match == "" && rule.Type == "" {
			return nil, fmt.Errorf("%s: rule %d has no match or type pattern", path, i+1)
		}
		if rule.Tag == "" {
			return nil, fmt.Errorf("%s: rule %d (%q) sets no tag", path, i+1, rule.Match)
		}
	}
	for i, t := range r.Transfer {
		if err := checkTransfer(t); err != nil {
			return nil, fmt.Errorf("%s: transfer %d: %w", path, i+1, err)
		}
	}
	return &r, nil
}

// checkTransfer rejects a half-written definition. Both sides are needed to
// pair anything at all, so a missing one is refused on load rather than
// silently matching nothing.
func checkTransfer(t Transfer) error {
	switch {
	case t.FromAccount == "":
		return fmt.Errorf("no from_account")
	case t.FromDesc == "":
		return fmt.Errorf("no from_desc pattern")
	case t.ToAccount == "":
		return fmt.Errorf("no to_account")
	case t.ToDesc == "":
		return fmt.Errorf("no to_desc pattern")
	}
	// A negative tolerance is a typo, and one at 100% or beyond would let any
	// amount pair with any other — at which point the dates decide alone, which
	// is the cross-currency rule and not something to arrive at by accident.
	if t.TolerancePct < 0 || t.TolerancePct >= 100 {
		return fmt.Errorf("tolerance_pct %g is not between 0 and 100", t.TolerancePct)
	}
	return nil
}

// AppendRule adds a rule to the end of rules.toml, creating the file if it is
// not there yet. Appending rather than inserting means an existing rule always
// keeps precedence, since the first match wins.
//
// The file is rewritten through a temporary file so a failure part-way cannot
// leave the user with a truncated config.
func AppendRule(root string, r Rule) error {
	if r.Match == "" && r.Type == "" {
		return fmt.Errorf("a rule needs a match or type pattern")
	}
	if r.Tag == "" {
		return fmt.Errorf("a rule needs a tag")
	}

	return appendBlock(root, formatRule(r))
}

// AppendTransfer adds a transfer definition to the end of rules.toml. Order
// matters for transfers as it does for rules: an earlier definition claims a
// transaction first, so appending cannot steal a leg from one already written.
func AppendTransfer(root string, t Transfer) error {
	if err := checkTransfer(t); err != nil {
		// A missing side reads better as what the form still wants; anything
		// else already says what is wrong with what was typed.
		if rest, ok := strings.CutPrefix(err.Error(), "no "); ok {
			return fmt.Errorf("a transfer needs %s", rest)
		}
		return err
	}
	return appendBlock(root, formatTransfer(t))
}

// appendBlock adds a rendered TOML table to the end of rules.toml, creating the
// file if it is not there yet.
//
// The file is rewritten through a temporary file so a failure part-way cannot
// leave the user with a truncated config.
func appendBlock(root, block string) error {
	path := filepath.Join(root, RulesFile)
	existing, err := os.ReadFile(path)
	if err != nil && !os.IsNotExist(err) {
		return fmt.Errorf("read %s: %w", path, err)
	}

	var b strings.Builder
	b.Write(existing)
	if len(existing) > 0 && !strings.HasSuffix(string(existing), "\n") {
		b.WriteString("\n")
	}
	b.WriteString("\n")
	b.WriteString(block)

	return writeFileAtomic(root, path, b.String())
}

// DeleteRules removes the rules at the given positions (0-based, as loaded by
// LoadRules) from rules.toml.
func DeleteRules(root string, positions []int) (int, error) {
	return deleteBlocks(root, "rule", positions)
}

// DeleteTransfers removes the transfer definitions at the given positions
// (0-based, as loaded by LoadRules) from rules.toml.
func DeleteTransfers(root string, positions []int) (int, error) {
	return deleteBlocks(root, "transfer", positions)
}

// blockStart is where one array-of-tables entry begins in rules.toml.
type blockStart struct {
	table string // "rule" or "transfer"
	line  int
}

// blockStarts finds every [[table]] header in the file. Every kind is
// collected, not just the one being deleted: a block ends where the *next*
// block of any kind begins, so deleting a rule that happens to sit above a
// transfer must not swallow it.
func blockStarts(lines []string) []blockStart {
	var out []blockStart
	for i, line := range lines {
		s := strings.TrimSpace(line)
		if strings.HasPrefix(s, "[[") && strings.HasSuffix(s, "]]") {
			out = append(out, blockStart{table: strings.TrimSpace(s[2 : len(s)-2]), line: i})
		}
	}
	return out
}

// deleteBlocks removes entries of one table from rules.toml.
//
// The file is edited textually rather than re-serialised from the parsed
// values, so comments, ordering and formatting the user put there by hand
// survive. A comment block sitting directly above a deleted entry goes with it,
// since it documents that entry; a comment separated by a blank line is treated
// as a section heading and left alone.
func deleteBlocks(root, table string, positions []int) (int, error) {
	if len(positions) == 0 {
		return 0, nil
	}
	doomed := map[int]bool{}
	for _, p := range positions {
		doomed[p] = true
	}

	path := filepath.Join(root, RulesFile)
	raw, err := os.ReadFile(path)
	if err != nil {
		return 0, fmt.Errorf("read %s: %w", path, err)
	}
	lines := strings.Split(string(raw), "\n")

	starts := blockStarts(lines)
	// mine[p] is where the p-th entry of this table sits among all the blocks.
	var mine []int
	for k, s := range starts {
		if s.table == table {
			mine = append(mine, k)
		}
	}
	for _, p := range positions {
		if p < 0 || p >= len(mine) {
			return 0, fmt.Errorf("%s %d is out of range; %s holds %d %ss",
				table, p+1, path, len(mine), table)
		}
	}

	// An entry owns the run of comment lines directly above it, with no blank
	// line in between. Anything further up is a heading for what follows.
	prefix := func(k int) int {
		i := starts[k].line
		for i > 0 && strings.HasPrefix(strings.TrimSpace(lines[i-1]), "#") {
			i--
		}
		return i
	}

	drop := map[int]bool{}
	for p := range doomed {
		k := mine[p]
		// The block runs up to the next block's comment prefix, so a comment
		// introducing the following one is not swept up with this one.
		end := len(lines)
		if k+1 < len(starts) {
			end = prefix(k + 1)
		}
		for i := prefix(k); i < end; i++ {
			drop[i] = true
		}
		// Blank lines are the gap between blocks, not part of either; leaving
		// them avoids gluing the neighbours together.
		for i := end - 1; i >= starts[k].line && strings.TrimSpace(lines[i]) == ""; i-- {
			delete(drop, i)
		}
	}

	kept := make([]string, 0, len(lines))
	for i, line := range lines {
		if !drop[i] {
			kept = append(kept, line)
		}
	}
	out := collapseBlankRuns(kept)

	// Never write something that will not load again, and never let deleting
	// one kind of block take a different kind with it.
	var check Rules
	if _, err := toml.Decode(out, &check); err != nil {
		return 0, fmt.Errorf("deleting from %s would produce invalid TOML: %w", path, err)
	}
	counts := map[string]int{"rule": len(check.Rule), "transfer": len(check.Transfer)}
	for _, kind := range []string{"rule", "transfer"} {
		want := 0
		for _, s := range starts {
			if s.table == kind {
				want++
			}
		}
		if kind == table {
			want -= len(doomed)
		}
		if counts[kind] != want {
			return 0, fmt.Errorf("deleting from %s would leave %d %ss, expected %d",
				path, counts[kind], kind, want)
		}
	}

	if err := writeFileAtomic(root, path, out); err != nil {
		return 0, err
	}
	return len(doomed), nil
}

// collapseBlankRuns squeezes the runs of blank lines that deletion leaves
// behind down to one.
func collapseBlankRuns(lines []string) string {
	out := make([]string, 0, len(lines))
	blank := false
	for _, line := range lines {
		if strings.TrimSpace(line) == "" {
			if blank {
				continue
			}
			blank = true
		} else {
			blank = false
		}
		out = append(out, line)
	}
	// Drop leading blank lines outright.
	for len(out) > 0 && strings.TrimSpace(out[0]) == "" {
		out = out[1:]
	}
	text := strings.Join(out, "\n")
	return strings.TrimRight(text, "\n") + "\n"
}

// writeFileAtomic replaces path via a temporary file in the same directory, so
// a failure part-way cannot truncate the user's config.
func writeFileAtomic(dir, path, content string) error {
	tmp, err := os.CreateTemp(dir, ".rules-*.toml")
	if err != nil {
		return fmt.Errorf("write %s: %w", path, err)
	}
	defer os.Remove(tmp.Name())

	if _, err := tmp.WriteString(content); err != nil {
		tmp.Close()
		return fmt.Errorf("write %s: %w", path, err)
	}
	if err := tmp.Close(); err != nil {
		return fmt.Errorf("write %s: %w", path, err)
	}
	if err := os.Chmod(tmp.Name(), 0o644); err != nil {
		return err
	}
	if err := os.Rename(tmp.Name(), path); err != nil {
		return fmt.Errorf("replace %s: %w", path, err)
	}
	return nil
}

// writeKey renders one TOML key, skipping it when empty. The column is wide
// enough for the longest key either block uses, so the values line up.
func writeKey(b *strings.Builder, key, value string) {
	if value != "" {
		fmt.Fprintf(b, "%-12s = %s\n", key, strconv.Quote(value))
	}
}

// formatTransfer renders a transfer as a TOML table, the two sides in the
// order money travels.
func formatTransfer(t Transfer) string {
	var b strings.Builder
	b.WriteString("[[transfer]]\n")
	writeKey(&b, "from_account", t.FromAccount)
	writeKey(&b, "from_desc", t.FromDesc)
	writeKey(&b, "to_account", t.ToAccount)
	writeKey(&b, "to_desc", t.ToDesc)
	// Only when set: a zero written out would suggest the key is doing something
	// when it is exactly the default every other definition already has.
	if t.TolerancePct != 0 {
		fmt.Fprintf(&b, "%-12s = %s\n", "tolerance_pct",
			strconv.FormatFloat(t.TolerancePct, 'f', -1, 64))
	}
	writeKey(&b, "note", t.Note)
	return b.String()
}

// formatRule renders a rule as a TOML table, omitting empty fields.
func formatRule(r Rule) string {
	var b strings.Builder
	b.WriteString("[[rule]]\n")
	write := func(key, value string) { writeKey(&b, key, value) }
	write("match", r.Match)
	write("type", r.Type)
	write("account", r.Account)
	write("tag", r.Tag)
	// Last, so the patterns and the tag stay lined up above it however long
	// the note runs.
	write("note", r.Note)
	return b.String()
}

// Account is a parsed account.toml.
type Account struct {
	Slug        string // folder name, filled in by LoadAccounts
	Dir         string // absolute path to the account folder
	Name        string `toml:"name"`
	Currency    string `toml:"currency"`
	MinorDigits *int   `toml:"minor_digits"`
	Parser      string `toml:"parser"`
	// Include restricts which files in the folder are treated as statements.
	// Defaults to every regular file except account.toml and dotfiles.
	Include []string `toml:"include"`
}

// Digits returns the configured minor-unit scale, defaulting to 2.
func (a *Account) Digits() int {
	if a.MinorDigits != nil {
		return *a.MinorDigits
	}
	return 2
}

// LoadAccounts finds every account folder under root. A folder is an account
// if it contains an account.toml.
func LoadAccounts(root string) ([]*Account, error) {
	entries, err := os.ReadDir(root)
	if err != nil {
		return nil, fmt.Errorf("read data root %s: %w", root, err)
	}
	var accounts []*Account
	for _, e := range entries {
		if !e.IsDir() || strings.HasPrefix(e.Name(), ".") {
			continue
		}
		dir := filepath.Join(root, e.Name())
		cfgPath := filepath.Join(dir, AccountFile)
		if _, err := os.Stat(cfgPath); err != nil {
			continue // not an account folder
		}
		a, err := loadAccount(dir, e.Name(), cfgPath)
		if err != nil {
			return nil, err
		}
		accounts = append(accounts, a)
	}
	sort.Slice(accounts, func(i, j int) bool { return accounts[i].Slug < accounts[j].Slug })
	return accounts, nil
}

func loadAccount(dir, slug, cfgPath string) (*Account, error) {
	var a Account
	if _, err := toml.DecodeFile(cfgPath, &a); err != nil {
		return nil, fmt.Errorf("%s: %w", cfgPath, err)
	}
	a.Slug = slug
	a.Dir = dir
	if a.Name == "" {
		a.Name = slug
	}
	if a.Currency == "" {
		return nil, fmt.Errorf("%s: currency is required", cfgPath)
	}
	if a.Parser == "" {
		return nil, fmt.Errorf("%s: parser is required", cfgPath)
	}
	if a.Digits() < 0 || a.Digits() > 8 {
		return nil, fmt.Errorf("%s: minor_digits must be between 0 and 8", cfgPath)
	}
	return &a, nil
}

// IndexPath returns the location of the SQLite index for a data root.
func IndexPath(root string) string {
	return filepath.Join(root, IndexFile)
}

// UserConfig is the small file in the user's config directory that says where
// the data root lives, so the tool can be run from anywhere without flags.
type UserConfig struct {
	Root string `toml:"root"`
}

// UserConfigPath returns the config file location, following the XDG base
// directory spec: $XDG_CONFIG_HOME/money/config.toml, falling back to
// ~/.config/money/config.toml.
func UserConfigPath() (string, error) {
	if dir := os.Getenv("XDG_CONFIG_HOME"); dir != "" {
		return filepath.Join(dir, "money", "config.toml"), nil
	}
	home, err := os.UserHomeDir()
	if err != nil {
		return "", fmt.Errorf("cannot locate the home directory: %w", err)
	}
	return filepath.Join(home, ".config", "money", "config.toml"), nil
}

// LoadUserConfig reads the config file. A missing file is not an error: it
// just means nothing overrides the default data root.
func LoadUserConfig() (*UserConfig, string, error) {
	path, err := UserConfigPath()
	if err != nil {
		return &UserConfig{}, "", err
	}
	var c UserConfig
	if _, err := toml.DecodeFile(path, &c); err != nil {
		if os.IsNotExist(err) {
			return &UserConfig{}, path, nil
		}
		return nil, path, fmt.Errorf("%s: %w", path, err)
	}
	return &c, path, nil
}

// RootSource records where a resolved data root came from, so the tool can
// explain itself when the path is not what the user expected.
type RootSource string

// The ways a data root can be chosen, in order of precedence.
const (
	RootFromFlag    RootSource = "--root flag"
	RootFromEnv     RootSource = "MONEY_ROOT"
	RootFromConfig  RootSource = "config file"
	RootFromDefault RootSource = "default"
)

// ResolveRoot decides which data root to use. An explicit flag wins, then
// $MONEY_ROOT, then the config file, then ~/money.
func ResolveRoot(flagRoot string) (root string, source RootSource, err error) {
	switch {
	case flagRoot != "":
		root, source = flagRoot, RootFromFlag
	default:
		if env := os.Getenv("MONEY_ROOT"); env != "" {
			root, source = env, RootFromEnv
		} else {
			c, _, err := LoadUserConfig()
			if err != nil {
				return "", "", err
			}
			if c.Root != "" {
				root, source = c.Root, RootFromConfig
			} else {
				home, err := os.UserHomeDir()
				if err != nil {
					return "", "", fmt.Errorf("cannot locate the home directory: %w", err)
				}
				root, source = filepath.Join(home, "money"), RootFromDefault
			}
		}
	}

	if root, err = expandHome(root); err != nil {
		return "", "", err
	}
	if root, err = filepath.Abs(root); err != nil {
		return "", "", err
	}
	return root, source, nil
}

// expandHome resolves a leading ~, which a hand-written config file is likely
// to contain and which the shell does not expand for us.
func expandHome(path string) (string, error) {
	if path != "~" && !strings.HasPrefix(path, "~/") {
		return path, nil
	}
	home, err := os.UserHomeDir()
	if err != nil {
		return "", fmt.Errorf("cannot expand %q: %w", path, err)
	}
	return filepath.Join(home, strings.TrimPrefix(strings.TrimPrefix(path, "~"), "/")), nil
}