package provider

// Amazon Bedrock Converse-Stream client.
//
// Endpoint: POST https://bedrock-runtime.{region}.amazonaws.com/model/{id}/converse-stream
//
// Auth: two paths supported.
//
//   - **Bearer token** via AWS_BEARER_TOKEN_BEDROCK. Modern, simple,
//     `bedrock:CallWithBearerToken` IAM permission required.
//   - **SigV4 signing** via AWS_ACCESS_KEY_ID + AWS_SECRET_ACCESS_KEY
//     (+ optional AWS_SESSION_TOKEN for temporary credentials, e.g. SSO
//     or IRSA-issued). Hand-rolled here so we don't pull in aws-sdk-go-v2.
//
// If both are present, bearer wins (simpler / fewer moving parts).
//
// Wire format: AWS event-stream binary framing. Each message is:
//
//	[4-byte total length BE]
//	[4-byte headers length BE]
//	[4-byte CRC32 of the 8-byte prelude]
//	[headers ...]      each = [1B name-length][name][1B value-type][2B value-length][value]
//	[payload ...]      payload-length = total - headers - 4 (trailer) - 12 (prelude+prelude-crc)
//	[4-byte CRC32 of message bytes minus trailer]
//
// We only consume the stream — never produce it — so we don't validate
// CRCs (the TLS+TCP transport already protects in-flight bytes). We do
// validate length-bounds to avoid OOB reads on a hostile / corrupted
// stream.
//
// Event types we care about (from `:event-type` header):
//   - messageStart        { role: "assistant" }
//   - contentBlockStart   { contentBlockIndex, start: { toolUse: { toolUseId, name } } }
//   - contentBlockDelta   { contentBlockIndex, delta: { text?, toolUse?: { input } } }
//   - contentBlockStop    { contentBlockIndex }
//   - messageStop         { stopReason }
//   - metadata            { usage: { inputTokens, outputTokens } }

import (
	"bytes"
	"context"
	"crypto/hmac"
	"crypto/sha256"
	"encoding/binary"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"os"
	"sort"
	"strings"
	"time"
)

type bedrockClient struct {
	// Auth mode is determined at construction time. Exactly one of
	// bearerToken or sigv4 is populated.
	bearerToken string
	sigv4       *bedrockSigV4Creds

	region  string
	baseURL string
	http    *http.Client
}

// bedrockSigV4Creds holds the IAM credentials used to sign each request.
type bedrockSigV4Creds struct {
	accessKeyID     string
	secretAccessKey string
	sessionToken    string // optional; STS / SSO / IRSA temp creds
}

// NewBedrockClient returns a Bedrock client.
//
// Auth resolution (first match wins):
//
//  1. apiKey == real bearer-ish string (not "<aws>") -> bearer route.
//  2. AWS_BEARER_TOKEN_BEDROCK env var -> bearer route.
//  3. AWS_ACCESS_KEY_ID + AWS_SECRET_ACCESS_KEY (+ optional
//     AWS_SESSION_TOKEN) -> SigV4 route.
//  4. AWS_PROFILE -> read ~/.aws/credentials, take that profile's keys.
//
// region defaults to us-east-1 unless AWS_REGION / AWS_DEFAULT_REGION is
// set or the baseURL embeds a region.
func NewBedrockClient(apiKey, baseURL string) Client {
	region := os.Getenv("AWS_REGION")
	if region == "" {
		region = os.Getenv("AWS_DEFAULT_REGION")
	}
	if region == "" {
		region = "us-east-1"
	}
	if baseURL == "" {
		baseURL = "https://bedrock-runtime." + region + ".amazonaws.com"
	}
	c := &bedrockClient{
		region:  region,
		baseURL: strings.TrimRight(baseURL, "/"),
		http:    &http.Client{Timeout: 0},
	}

	// Bearer route.
	token := apiKey
	if token == "" || token == "<aws>" {
		token = os.Getenv("AWS_BEARER_TOKEN_BEDROCK")
	}
	if token != "" && token != "<aws>" {
		c.bearerToken = token
		return c
	}

	// SigV4 route: env vars first.
	ak := os.Getenv("AWS_ACCESS_KEY_ID")
	sk := os.Getenv("AWS_SECRET_ACCESS_KEY")
	st := os.Getenv("AWS_SESSION_TOKEN")
	if ak != "" && sk != "" {
		c.sigv4 = &bedrockSigV4Creds{accessKeyID: ak, secretAccessKey: sk, sessionToken: st}
		return c
	}

	// SigV4 route: ~/.aws/credentials via AWS_PROFILE.
	if profile := os.Getenv("AWS_PROFILE"); profile != "" {
		if creds, err := readAWSCredentialsFile(profile); err == nil {
			c.sigv4 = creds
			return c
		}
	}

	return &unimplementedClient{
		name: "amazon-bedrock",
		hint: "no Bedrock credentials found (set AWS_BEARER_TOKEN_BEDROCK, AWS_ACCESS_KEY_ID+AWS_SECRET_ACCESS_KEY, or AWS_PROFILE)",
	}
}

// readAWSCredentialsFile parses ~/.aws/credentials and returns the
// access-key/secret-key (and optional session-token) for the named
// profile. The file is an INI-like format with `[profile]` headers.
func readAWSCredentialsFile(profile string) (*bedrockSigV4Creds, error) {
	home, err := os.UserHomeDir()
	if err != nil {
		return nil, err
	}
	b, err := os.ReadFile(home + "/.aws/credentials")
	if err != nil {
		return nil, err
	}
	var current string
	creds := map[string]map[string]string{}
	for _, line := range strings.Split(string(b), "\n") {
		line = strings.TrimSpace(line)
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}
		if strings.HasPrefix(line, "[") && strings.HasSuffix(line, "]") {
			current = strings.TrimSpace(line[1 : len(line)-1])
			creds[current] = map[string]string{}
			continue
		}
		if current == "" {
			continue
		}
		eq := strings.IndexByte(line, '=')
		if eq < 0 {
			continue
		}
		k := strings.TrimSpace(line[:eq])
		v := strings.TrimSpace(line[eq+1:])
		creds[current][k] = v
	}
	p, ok := creds[profile]
	if !ok {
		return nil, fmt.Errorf("aws profile %q not found in ~/.aws/credentials", profile)
	}
	ak := p["aws_access_key_id"]
	sk := p["aws_secret_access_key"]
	if ak == "" || sk == "" {
		return nil, fmt.Errorf("aws profile %q missing aws_access_key_id or aws_secret_access_key", profile)
	}
	return &bedrockSigV4Creds{
		accessKeyID:     ak,
		secretAccessKey: sk,
		sessionToken:    p["aws_session_token"],
	}, nil
}

func (c *bedrockClient) Name() string { return "amazon-bedrock" }

// ---- request building ----

type bedrockMessage struct {
	Role    string                   `json:"role"`
	Content []map[string]interface{} `json:"content"`
}

type bedrockToolSpec struct {
	ToolSpec struct {
		Name        string `json:"name"`
		Description string `json:"description,omitempty"`
		InputSchema struct {
			JSON json.RawMessage `json:"json"`
		} `json:"inputSchema"`
	} `json:"toolSpec"`
}

type bedrockRequest struct {
	Messages        []bedrockMessage    `json:"messages"`
	System          []map[string]string `json:"system,omitempty"`
	InferenceConfig struct {
		MaxTokens   int      `json:"maxTokens,omitempty"`
		Temperature *float32 `json:"temperature,omitempty"`
	} `json:"inferenceConfig,omitempty"`
	ToolConfig *struct {
		Tools []bedrockToolSpec `json:"tools"`
	} `json:"toolConfig,omitempty"`
}

func (c *bedrockClient) buildRequest(req Request) (*bedrockRequest, error) {
	out := &bedrockRequest{}
	if req.System != "" {
		out.System = []map[string]string{{"text": req.System}}
	}
	out.InferenceConfig.Temperature = req.Temperature
	out.InferenceConfig.MaxTokens = req.MaxTokens
	if out.InferenceConfig.MaxTokens == 0 {
		out.InferenceConfig.MaxTokens = 4096
	}
	for _, m := range req.Messages {
		role := string(m.Role)
		if role == "tool" {
			role = "user"
		}
		bm := bedrockMessage{Role: role}
		for _, c := range m.Content {
			switch v := c.(type) {
			case TextBlock:
				bm.Content = append(bm.Content, map[string]interface{}{"text": v.Text})
			case ToolCallBlock:
				var input map[string]interface{}
				_ = json.Unmarshal(v.Arguments, &input)
				if input == nil {
					input = map[string]interface{}{}
				}
				bm.Content = append(bm.Content, map[string]interface{}{
					"toolUse": map[string]interface{}{
						"toolUseId": v.ID, "name": v.Name, "input": input,
					},
				})
			case ToolResultBlock:
				var resultContent []map[string]interface{}
				for _, inner := range v.Content {
					if tb, ok := inner.(TextBlock); ok {
						resultContent = append(resultContent, map[string]interface{}{"text": tb.Text})
					}
				}
				status := "success"
				if v.IsError {
					status = "error"
				}
				bm.Content = append(bm.Content, map[string]interface{}{
					"toolResult": map[string]interface{}{
						"toolUseId": v.CallID,
						"content":   resultContent,
						"status":    status,
					},
				})
			}
		}
		out.Messages = append(out.Messages, bm)
	}
	if len(req.Tools) > 0 {
		tc := struct {
			Tools []bedrockToolSpec `json:"tools"`
		}{}
		for _, t := range req.Tools {
			var ts bedrockToolSpec
			ts.ToolSpec.Name = t.Name
			ts.ToolSpec.Description = t.Description
			ts.ToolSpec.InputSchema.JSON = t.Schema
			tc.Tools = append(tc.Tools, ts)
		}
		out.ToolConfig = &tc
	}
	return out, nil
}

func (c *bedrockClient) Stream(ctx context.Context, req Request) (<-chan Event, error) {
	wire, err := c.buildRequest(req)
	if err != nil {
		return nil, err
	}
	body, err := json.Marshal(wire)
	if err != nil {
		return nil, err
	}
	url := c.baseURL + "/model/" + req.Model + "/converse-stream"
	httpReq, err := http.NewRequestWithContext(ctx, "POST", url, bytes.NewReader(body))
	if err != nil {
		return nil, err
	}
	httpReq.Header.Set("content-type", "application/json")
	httpReq.Header.Set("accept", "application/vnd.amazon.eventstream")
	if c.bearerToken != "" {
		httpReq.Header.Set("authorization", "Bearer "+c.bearerToken)
	} else if c.sigv4 != nil {
		if err := signSigV4(httpReq, body, "bedrock", c.region, c.sigv4, time.Now().UTC()); err != nil {
			return nil, fmt.Errorf("bedrock: sigv4 sign: %w", err)
		}
	} else {
		return nil, fmt.Errorf("bedrock: no auth configured")
	}
	resp, err := c.http.Do(httpReq)
	if err != nil {
		return nil, fmt.Errorf("bedrock: %w", err)
	}
	if resp.StatusCode != http.StatusOK {
		b, _ := io.ReadAll(resp.Body)
		resp.Body.Close()
		msg := strings.TrimSpace(string(b))
		// A 403 on the bearer route is almost always a region mismatch:
		// short-term Bedrock API keys are scoped to the region of the
		// console session that minted them, but zot defaults to
		// us-east-1. Surface the resolved region and the fix so the user
		// is not left guessing why a freshly-copied key is "invalid".
		if resp.StatusCode == http.StatusForbidden && c.bearerToken != "" {
			return nil, fmt.Errorf(
				"bedrock: http 403 (region=%s): %s\nhint: Bedrock API keys are region-scoped. If your key was created in another region, set AWS_REGION (e.g. AWS_REGION=eu-central-1) or pass --base-url https://bedrock-runtime.<region>.amazonaws.com",
				c.region, msg)
		}
		return nil, fmt.Errorf("bedrock: http %d: %s", resp.StatusCode, msg)
	}
	out := make(chan Event, 16)
	go c.runStream(ctx, resp, req, out)
	return out, nil
}

func (c *bedrockClient) runStream(ctx context.Context, resp *http.Response, req Request, out chan<- Event) {
	defer close(out)
	defer resp.Body.Close()

	out <- EventStart{Provider: "amazon-bedrock", Model: req.Model}

	// State accumulated across deltas.
	contentBlocks := map[int]*bedrockBlockState{}
	stop := StopEnd
	finalMsg := Message{Role: RoleAssistant, Time: time.Now()}
	var usage Usage

	for {
		if ctx.Err() != nil {
			stop = StopAborted
			break
		}
		evt, err := readEventStreamMessage(resp.Body)
		if err != nil {
			if err == io.EOF {
				break
			}
			out <- EventDone{Stop: StopError, Err: err, Message: finalMsg}
			return
		}
		eventType := evt.headerString(":event-type")
		messageType := evt.headerString(":message-type")
		if messageType == "exception" {
			out <- EventDone{Stop: StopError, Err: fmt.Errorf("bedrock exception (%s): %s", evt.headerString(":exception-type"), string(evt.payload)), Message: finalMsg}
			return
		}
		switch eventType {
		case "messageStart":
			// nothing to do; role is always "assistant"
		case "contentBlockStart":
			var d struct {
				ContentBlockIndex int `json:"contentBlockIndex"`
				Start             struct {
					ToolUse *struct {
						ToolUseID string `json:"toolUseId"`
						Name      string `json:"name"`
					} `json:"toolUse"`
				} `json:"start"`
			}
			if err := json.Unmarshal(evt.payload, &d); err != nil {
				continue
			}
			st := &bedrockBlockState{}
			contentBlocks[d.ContentBlockIndex] = st
			if d.Start.ToolUse != nil {
				st.isToolUse = true
				st.toolID = d.Start.ToolUse.ToolUseID
				st.toolName = d.Start.ToolUse.Name
				out <- EventToolStart{ID: st.toolID, Name: st.toolName}
			}
		case "contentBlockDelta":
			var d struct {
				ContentBlockIndex int `json:"contentBlockIndex"`
				Delta             struct {
					Text    string `json:"text"`
					ToolUse *struct {
						Input string `json:"input"`
					} `json:"toolUse"`
				} `json:"delta"`
			}
			if err := json.Unmarshal(evt.payload, &d); err != nil {
				continue
			}
			st := contentBlocks[d.ContentBlockIndex]
			if st == nil {
				continue
			}
			if d.Delta.Text != "" {
				st.text.WriteString(d.Delta.Text)
				out <- EventTextDelta{Delta: d.Delta.Text}
			}
			if d.Delta.ToolUse != nil && d.Delta.ToolUse.Input != "" {
				st.toolArgs.WriteString(d.Delta.ToolUse.Input)
				out <- EventToolArgs{ID: st.toolID, Delta: d.Delta.ToolUse.Input}
			}
		case "contentBlockStop":
			var d struct {
				ContentBlockIndex int `json:"contentBlockIndex"`
			}
			if err := json.Unmarshal(evt.payload, &d); err != nil {
				continue
			}
			st := contentBlocks[d.ContentBlockIndex]
			if st == nil {
				continue
			}
			if st.isToolUse {
				args := st.toolArgs.String()
				if args == "" {
					args = "{}"
				}
				finalMsg.Content = append(finalMsg.Content, ToolCallBlock{
					ID: st.toolID, Name: st.toolName, Arguments: json.RawMessage(args),
				})
				out <- EventToolEnd{ID: st.toolID}
			} else if st.text.Len() > 0 {
				finalMsg.Content = append(finalMsg.Content, TextBlock{Text: st.text.String()})
			}
		case "messageStop":
			var d struct {
				StopReason string `json:"stopReason"`
			}
			_ = json.Unmarshal(evt.payload, &d)
			switch d.StopReason {
			case "tool_use":
				stop = StopToolUse
			case "end_turn":
				stop = StopEnd
			case "max_tokens":
				stop = StopLength
			default:
				stop = StopEnd
			}
		case "metadata":
			var d struct {
				Usage struct {
					InputTokens           int `json:"inputTokens"`
					OutputTokens          int `json:"outputTokens"`
					CacheReadInputTokens  int `json:"cacheReadInputTokens"`
					CacheWriteInputTokens int `json:"cacheWriteInputTokens"`
				} `json:"usage"`
			}
			if err := json.Unmarshal(evt.payload, &d); err == nil {
				usage.InputTokens = d.Usage.InputTokens
				usage.OutputTokens = d.Usage.OutputTokens
				usage.CacheReadTokens = d.Usage.CacheReadInputTokens
				usage.CacheWriteTokens = d.Usage.CacheWriteInputTokens
				if m, err := FindModel("amazon-bedrock", req.Model); err == nil {
					usage.CostUSD = ComputeCost(m, usage)
				}
				out <- EventUsage{Usage: usage}
			}
		}
	}
	out <- EventDone{Stop: stop, Message: finalMsg}
}

type bedrockBlockState struct {
	isToolUse bool
	toolID    string
	toolName  string
	toolArgs  strings.Builder
	text      strings.Builder
}

// ---- event-stream binary framing parser ----

type eventStreamMessage struct {
	headers map[string]string
	payload []byte
}

func (m *eventStreamMessage) headerString(name string) string { return m.headers[name] }

// readEventStreamMessage reads one event-stream message from r. Returns
// io.EOF when the stream is finished cleanly.
func readEventStreamMessage(r io.Reader) (*eventStreamMessage, error) {
	var prelude [8]byte
	if _, err := io.ReadFull(r, prelude[:]); err != nil {
		return nil, err
	}
	totalLen := binary.BigEndian.Uint32(prelude[0:4])
	headersLen := binary.BigEndian.Uint32(prelude[4:8])
	if totalLen < 16 || headersLen > totalLen-16 {
		return nil, fmt.Errorf("bedrock: malformed event-stream prelude (total=%d headers=%d)", totalLen, headersLen)
	}
	// Skip prelude CRC (4 bytes); we don't validate.
	var preludeCRC [4]byte
	if _, err := io.ReadFull(r, preludeCRC[:]); err != nil {
		return nil, err
	}
	headersBuf := make([]byte, headersLen)
	if _, err := io.ReadFull(r, headersBuf); err != nil {
		return nil, err
	}
	payloadLen := int(totalLen) - 16 - int(headersLen)
	payload := make([]byte, payloadLen)
	if _, err := io.ReadFull(r, payload); err != nil {
		return nil, err
	}
	var trailer [4]byte
	if _, err := io.ReadFull(r, trailer[:]); err != nil {
		return nil, err
	}
	// Parse headers.
	hdrs := map[string]string{}
	i := 0
	for i < len(headersBuf) {
		if i+1 > len(headersBuf) {
			break
		}
		nameLen := int(headersBuf[i])
		i++
		if i+nameLen > len(headersBuf) {
			return nil, fmt.Errorf("bedrock: header name overflow")
		}
		name := string(headersBuf[i : i+nameLen])
		i += nameLen
		if i+1 > len(headersBuf) {
			return nil, fmt.Errorf("bedrock: header value-type missing")
		}
		valueType := headersBuf[i]
		i++
		// Type 7 = string with 2-byte length. Type 6 = byte-array similar.
		// Other types (bool/int/long/timestamp/uuid) we treat as opaque
		// strings since bedrock's event-type headers are always type 7.
		switch valueType {
		case 7, 6:
			if i+2 > len(headersBuf) {
				return nil, fmt.Errorf("bedrock: header value length missing")
			}
			vLen := int(binary.BigEndian.Uint16(headersBuf[i : i+2]))
			i += 2
			if i+vLen > len(headersBuf) {
				return nil, fmt.Errorf("bedrock: header value overflow")
			}
			hdrs[name] = string(headersBuf[i : i+vLen])
			i += vLen
		case 0: // bool true
			hdrs[name] = "true"
		case 1: // bool false
			hdrs[name] = "false"
		case 2: // byte
			if i+1 > len(headersBuf) {
				return nil, fmt.Errorf("bedrock: header byte missing")
			}
			i++
		case 3: // int16
			if i+2 > len(headersBuf) {
				return nil, fmt.Errorf("bedrock: header int16 missing")
			}
			i += 2
		case 4: // int32
			if i+4 > len(headersBuf) {
				return nil, fmt.Errorf("bedrock: header int32 missing")
			}
			i += 4
		case 5: // int64
			if i+8 > len(headersBuf) {
				return nil, fmt.Errorf("bedrock: header int64 missing")
			}
			i += 8
		case 8: // timestamp (int64)
			if i+8 > len(headersBuf) {
				return nil, fmt.Errorf("bedrock: header timestamp missing")
			}
			i += 8
		case 9: // uuid (16 bytes)
			if i+16 > len(headersBuf) {
				return nil, fmt.Errorf("bedrock: header uuid missing")
			}
			i += 16
		default:
			return nil, fmt.Errorf("bedrock: unknown header value-type %d", valueType)
		}
	}
	return &eventStreamMessage{headers: hdrs, payload: payload}, nil
}

// ---- SigV4 signing ----
//
// AWS Signature Version 4. Implemented from scratch so we don't pull in
// aws-sdk-go-v2 (large transitive dep tree just for one signer). The
// algorithm is documented at:
//   https://docs.aws.amazon.com/general/latest/gr/sigv4-signed-request-examples.html
//
// Steps:
//
//   1. Build the canonical request: method + path + canonical query +
//      canonical headers + signed headers + payload-hash.
//   2. Build the string-to-sign: algorithm + amz-date + credential scope +
//      sha256(canonical request).
//   3. Derive the signing key by chaining HMAC-SHA256:
//      kSecret -> kDate -> kRegion -> kService -> kSigning
//   4. signature = HMAC-SHA256(kSigning, stringToSign)
//   5. Authorization header: AWS4-HMAC-SHA256 Credential=..., SignedHeaders=..., Signature=...
//
// Payload hash is sha256 of the JSON body. For Bedrock's streaming
// endpoint this works because the request body is fully buffered before
// the call — we don't need the streaming-payload variant.

func sigHMAC(key, msg []byte) []byte {
	m := hmac.New(sha256.New, key)
	m.Write(msg)
	return m.Sum(nil)
}

func sigSHA256Hex(b []byte) string {
	h := sha256.Sum256(b)
	return hex.EncodeToString(h[:])
}

func signSigV4(req *http.Request, payload []byte, service, region string, creds *bedrockSigV4Creds, now time.Time) error {
	amzDate := now.Format("20060102T150405Z")
	shortDate := now.Format("20060102")
	credentialScope := shortDate + "/" + region + "/" + service + "/aws4_request"

	// Required headers.
	req.Header.Set("host", req.URL.Host)
	req.Header.Set("x-amz-date", amzDate)
	if creds.sessionToken != "" {
		req.Header.Set("x-amz-security-token", creds.sessionToken)
	}
	payloadHash := sigSHA256Hex(payload)
	req.Header.Set("x-amz-content-sha256", payloadHash)

	// Canonical headers: lowercase name, trimmed value, sorted by name,
	// each on its own line ending with \n.
	var names []string
	values := map[string]string{}
	for k, vs := range req.Header {
		lower := strings.ToLower(k)
		// Skip authorization (we're computing it) and any
		// connection-specific headers.
		if lower == "authorization" {
			continue
		}
		names = append(names, lower)
		values[lower] = strings.Join(vs, ",")
	}
	// host is special — it's not in req.Header by default; set above.
	sort.Strings(names)
	var canonHdrs strings.Builder
	for _, n := range names {
		canonHdrs.WriteString(n)
		canonHdrs.WriteByte(':')
		canonHdrs.WriteString(strings.TrimSpace(values[n]))
		canonHdrs.WriteByte('\n')
	}
	signedHeaders := strings.Join(names, ";")

	// Canonical query string: keys sorted by name, each key and value
	// URI-escaped (RFC 3986).
	q := req.URL.Query()
	var qKeys []string
	for k := range q {
		qKeys = append(qKeys, k)
	}
	sort.Strings(qKeys)
	var canonQuery strings.Builder
	for i, k := range qKeys {
		if i > 0 {
			canonQuery.WriteByte('&')
		}
		canonQuery.WriteString(url.QueryEscape(k))
		canonQuery.WriteByte('=')
		canonQuery.WriteString(url.QueryEscape(q.Get(k)))
	}

	// Canonical request.
	canonReq := req.Method + "\n" +
		canonicalSigV4Path(req.URL.Path) + "\n" +
		canonQuery.String() + "\n" +
		canonHdrs.String() + "\n" +
		signedHeaders + "\n" +
		payloadHash

	// String to sign.
	stringToSign := "AWS4-HMAC-SHA256\n" + amzDate + "\n" + credentialScope + "\n" + sigSHA256Hex([]byte(canonReq))

	// Signing key.
	kDate := sigHMAC([]byte("AWS4"+creds.secretAccessKey), []byte(shortDate))
	kRegion := sigHMAC(kDate, []byte(region))
	kService := sigHMAC(kRegion, []byte(service))
	kSigning := sigHMAC(kService, []byte("aws4_request"))

	// Signature.
	signature := hex.EncodeToString(sigHMAC(kSigning, []byte(stringToSign)))

	// Authorization header.
	auth := "AWS4-HMAC-SHA256 " +
		"Credential=" + creds.accessKeyID + "/" + credentialScope + ", " +
		"SignedHeaders=" + signedHeaders + ", " +
		"Signature=" + signature
	req.Header.Set("Authorization", auth)
	return nil
}

// canonicalSigV4Path returns the path with each segment URI-encoded
// according to SigV4 rules (everything except unreserved chars
// A-Za-z0-9-_.~ and / between segments). Bedrock model IDs contain
// dots, colons, and slashes; the colons must be percent-encoded.
func canonicalSigV4Path(p string) string {
	if p == "" {
		return "/"
	}
	segments := strings.Split(p, "/")
	for i, s := range segments {
		segments[i] = sigV4EncodeSegment(s)
	}
	return strings.Join(segments, "/")
}

func sigV4EncodeSegment(s string) string {
	var b strings.Builder
	for i := 0; i < len(s); i++ {
		c := s[i]
		// Unreserved per RFC 3986.
		if (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || (c >= '0' && c <= '9') ||
			c == '-' || c == '_' || c == '.' || c == '~' {
			b.WriteByte(c)
			continue
		}
		fmt.Fprintf(&b, "%%%02X", c)
	}
	return b.String()
}