// Package gateway is the model gateway: the one place in this service that
// talks to a language model.
//
// Everything else — the runtime loop, the tool layer, retrieval — reaches a
// model through this package and nowhere else. That is the whole point of it
// being a layer rather than a helper:
//
//   - **Routing lives here.** An agent spec declares a `reasoning` tier, not a
//     model id. Which model and how much thinking that tier buys is a
//     deployment decision, and it changes without touching a single spec.
//   - **Token accounting lives here.** Every call returns what it cost. A
//     budget the runtime cannot measure is a budget it cannot enforce, and
//     I3 requires it to enforce one.
//   - **Refusal is an outcome, not an exception.** A model that declines comes
//     back as a structured Refused, which is one of the six termination
//     reasons the runtime already knows how to end a run with.
//
// What this package deliberately does *not* do: assemble prompts, decide what
// a caller may read, or loop. It sends one request and reports one result.
// Composition is the runtime's job and authorization is the tool layer's, and
// folding either of them in here would put policy behind a transport.
package gateway

import (
	"context"
	"encoding/json"
	"fmt"
	"strings"
)

// Tier is an agent spec's `reasoning` value.
//
// Three tiers, because an author choosing between "fast" and "deep" is making
// a judgement about the work, not about a model. The mapping from a tier to a
// model and an effort level is this package's business and is configured per
// deployment — a spec that named a model directly would pin every tenant to
// whatever was current the day it was written.
type Tier string

const (
	TierFast     Tier = "fast"
	TierBalanced Tier = "balanced"
	TierDeep     Tier = "deep"
)

// DefaultTier is what a spec that declares no reasoning mode gets. It matches
// the frontend vocabulary's own default, so a definition means the same thing
// on both sides of the wire.
const DefaultTier = TierBalanced

// ParseTier resolves a spec's declared reasoning value.
//
// An unrecognised tier falls back rather than failing: the tier affects how
// much a turn costs, never whether it is allowed, so refusing the run would
// turn a typo in a definition into an outage. The caller is told, so a
// definition that has drifted from the vocabulary is still visible.
func ParseTier(raw string) (Tier, bool) {
	switch Tier(strings.ToLower(strings.TrimSpace(raw))) {
	case TierFast:
		return TierFast, true
	case TierBalanced:
		return TierBalanced, true
	case TierDeep:
		return TierDeep, true
	case "":
		return DefaultTier, true
	default:
		return DefaultTier, false
	}
}

// Role is who said something.
type Role string

const (
	RoleUser      Role = "user"
	RoleAssistant Role = "assistant"
)

// ToolCall is the model asking for a tool to be run.
type ToolCall struct {
	// ID correlates the call with its result. Echoed back verbatim: it is the
	// model's own handle, and a result carrying a different one is a result
	// attached to the wrong question.
	ID    string
	Name  string
	Input json.RawMessage

	// Extra is provider metadata attached to the call, carried back to the
	// provider verbatim on the next turn and never read here.
	//
	// It exists because at least one provider requires it. Gemini 3 models
	// attach a "thought signature" to every function call and REJECT the
	// follow-up request — 400, "Function call is missing a thought_signature"
	// — if the assistant message that echoes the call does not carry it back.
	// A gateway that rebuilds the assistant turn from ID, Name and Input alone
	// drops it, and every tool-using run dies on its second model call while
	// the first one looked perfectly healthy. That is exactly what happened
	// on 2026-09-22 when production was pointed at Gemini.
	//
	// The gateway does not know what is in it and must not: the whole point
	// of speaking one wire shape is that a vendor's private fields pass
	// through untouched. It is the raw JSON of the call's extra_content
	// object, or nil when the provider sent none, in which case it is omitted
	// from the request again.
	Extra json.RawMessage
}

// ToolResult is what came back, on its way to the model.
//
// Content is a string because that is what crosses the wire, but it carries
// encoded structured data — §4 keeps formatting the model's job, so a handler
// never writes prose and this never carries any.
type ToolResult struct {
	CallID  string
	Content string
	IsError bool
}

// Message is one turn of a conversation.
//
// A turn is text, or tool calls, or tool results — an assistant turn may carry
// text and calls together, which is why these are fields rather than a union.
type Message struct {
	Role        Role
	Text        string
	ToolCalls   []ToolCall
	ToolResults []ToolResult
}

// ToolDef is a tool as the model sees it.
//
// Deliberately not the tool layer's own type. The gateway must not import the
// tool package: a model provider knowing what an `effect` or a confirmation
// token is would put policy behind a transport, and the confirmation gate has
// to sit where the model cannot reach it.
type ToolDef struct {
	Name        string
	Description string
	InputSchema map[string]any
}

// Request is one model call.
type Request struct {
	// Tier selects the model and effort. From the agent spec.
	Tier Tier

	// System is the assembled system prompt.
	//
	// I7: retrieved document text must never reach this field. Retrieved
	// content belongs in a delimited context block inside a user message,
	// where the system prompt has already said that its contents are data.
	// Nothing here can enforce that — it is a property of what the runtime
	// passes — so it is stated where the field is declared.
	System string

	// Messages is the conversation so far, oldest first.
	Messages []Message

	// Tools the model may call this turn. Order matters: it is part of the
	// cached prefix, so the caller sorts it once and keeps it stable.
	Tools []ToolDef

	// MaxOutputTokens caps this response. Zero takes the configured default.
	//
	// This is a hard ceiling the model is not aware of, so it truncates rather
	// than winding down. It is not the run's token budget — that is the
	// runtime's, and it spans every call in a run.
	MaxOutputTokens int64
}

// Usage is what a call cost.
type Usage struct {
	InputTokens         int64
	OutputTokens        int64
	CacheReadTokens     int64
	CacheCreationTokens int64
}

// Total is every token this call is billed for.
//
// Cache reads are counted: they are cheaper than fresh input, not free, and a
// budget that ignored them would drift further from the truth the longer a
// conversation ran — which is exactly when it matters most.
func (u Usage) Total() int64 {
	return u.InputTokens + u.OutputTokens + u.CacheReadTokens + u.CacheCreationTokens
}

// Response is one model reply.
type Response struct {
	Text string

	// ToolCalls the model wants run before it can continue. Non-empty exactly
	// when StopReason is "tool_use".
	ToolCalls []ToolCall

	StopReason string
	Usage      Usage

	// Model is the id actually used, not the tier that was asked for. Logged
	// with every run so a change of routing is visible in the trajectory
	// rather than inferred from a deploy date.
	Model string
	Tier  Tier
}

// Error codes. Structured rather than bare strings, per §10 — user-facing text
// is derived at the surface layer, never raised from here.
const (
	CodeNotConfigured  = "gateway.not_configured"
	CodeInvalidRequest = "gateway.invalid_request"
	CodeUnauthorized   = "gateway.unauthorized"
	CodeRateLimited    = "gateway.rate_limited"
	CodeTimeout        = "gateway.timeout"
	CodeRefused        = "gateway.refused"
	CodeUpstream       = "gateway.upstream"
)

// Error is a gateway failure with a code the runtime can branch on.
type Error struct {
	Code    string
	Message string

	// Status is the upstream HTTP status, when there was one.
	Status int

	// Category carries a refusal's reason when Code is CodeRefused. An open
	// set upstream, so it is a string and is never switched on exhaustively.
	Category string

	Cause error
}

func (e *Error) Error() string {
	if e.Status != 0 {
		return fmt.Sprintf("%s: %s (http %d)", e.Code, e.Message, e.Status)
	}
	return fmt.Sprintf("%s: %s", e.Code, e.Message)
}

func (e *Error) Unwrap() error { return e.Cause }

// Retryable reports whether the same request could succeed if sent again.
//
// The runtime needs this to decide between a retry and a terminal
// ToolFailure. A refusal is emphatically not retryable — re-sending a request
// the model declined is how a loop burns a whole budget on one turn.
func (e *Error) Retryable() bool {
	switch e.Code {
	case CodeRateLimited, CodeTimeout:
		return true
	case CodeUpstream:
		return e.Status >= 500
	default:
		return false
	}
}

// Streamer is a Gateway that can deliver text as it arrives.
//
// A SEPARATE interface, not a method on Gateway, and that is deliberate. Adding
// Stream to Gateway would break every fake in the test suite and force each one
// to implement a transport it does not care about — and those fakes exist to
// test the LOOP, not the wire. StreamComplete bridges the two, so a caller
// writes one line and gets streaming wherever it is available.
type Streamer interface {
	// Stream calls the model, invoking onDelta with each fragment of assistant
	// text. Tool calls are NOT streamed: a partially-built argument object is a
	// different object from the finished one, and usually an invalid one.
	Stream(ctx context.Context, req Request, onDelta func(string)) (*Response, error)
}

// Gateway is the model boundary.
//
// One method. A second implementation — a fake for tests, a recorded one for
// evals — has one thing to satisfy, which is what keeps the eval harness from
// needing a network.
type Gateway interface {
	Complete(ctx context.Context, req Request) (*Response, error)
}

// Validate checks a request before it costs anything.
func (r Request) Validate() error {
	if len(r.Messages) == 0 {
		return &Error{Code: CodeInvalidRequest, Message: "a request needs at least one message"}
	}
	for i, m := range r.Messages {
		if m.Role != RoleUser && m.Role != RoleAssistant {
			return &Error{
				Code:    CodeInvalidRequest,
				Message: fmt.Sprintf("messages[%d]: %q is not a role", i, m.Role),
			}
		}
		// A turn must say something, but "something" is text, tool calls or
		// tool results. A tool-result turn legitimately carries no text at all.
		if strings.TrimSpace(m.Text) == "" && len(m.ToolCalls) == 0 && len(m.ToolResults) == 0 {
			return &Error{
				Code:    CodeInvalidRequest,
				Message: fmt.Sprintf("messages[%d]: a message cannot be empty", i),
			}
		}
	}
	for i, t := range r.Tools {
		if strings.TrimSpace(t.Name) == "" {
			return &Error{Code: CodeInvalidRequest, Message: fmt.Sprintf("tools[%d]: a tool needs a name", i)}
		}
		if strings.TrimSpace(t.Description) == "" {
			// The description is what the model reads instead of documentation.
			return &Error{Code: CodeInvalidRequest, Message: fmt.Sprintf("tools[%d]: %s has no description", i, t.Name)}
		}
	}
	return nil
}

// StreamComplete runs a request through whichever path the gateway supports.
//
// A gateway that cannot stream is not a broken gateway — every fake in the test
// suite is one, and so is any future provider without a streaming API. Falling
// back to Complete and delivering the finished text as a single delta keeps the
// caller's code identical either way, which is what stops streaming from
// becoming a second code path through the loop.
func StreamComplete(ctx context.Context, gw Gateway, req Request, onDelta func(string)) (*Response, error) {
	// Normalised once, here, so no implementation has to guard it. A caller
	// that does not want deltas passes nil — every eval and every test does —
	// and an implementation that took that literally would panic on the first
	// fragment. Making each Streamer remember the check is how one of them
	// eventually forgets.
	if onDelta == nil {
		onDelta = func(string) {}
	}
	if s, ok := gw.(Streamer); ok {
		return s.Stream(ctx, req, onDelta)
	}
	resp, err := gw.Complete(ctx, req)
	if err == nil && resp != nil && resp.Text != "" && onDelta != nil {
		onDelta(resp.Text)
	}
	return resp, err
}