// Copyright 2023, Google Inc.
// All rights reserved.
//
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// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
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// contributors may be used to endorse or promote products derived from
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//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

// Package callctx provides helpers for storing and retrieving values out of
// [context.Context]. These values are used by our client libraries in various
// ways across the stack.
package callctx

import (
	"context"
	"fmt"
)

const (
	// XGoogFieldMaskHeader is the canonical header key for the [System Parameter]
	// that specifies the response read mask. The value(s) for this header
	// must adhere to format described in [fieldmaskpb].
	//
	// [System Parameter]: https://cloud.google.com/apis/docs/system-parameters
	// [fieldmaskpb]: https://google.golang.org/protobuf/types/known/fieldmaskpb
	XGoogFieldMaskHeader = "x-goog-fieldmask"

	headerKey = contextKey("header")
)

// contextKey is a private type used to store/retrieve context values.
type contextKey string

// HeadersFromContext retrieves headers set from [SetHeaders]. These headers
// can then be cast to http.Header or metadata.MD to send along on requests.
func HeadersFromContext(ctx context.Context) map[string][]string {
	m, ok := ctx.Value(headerKey).(map[string][]string)
	if !ok {
		return nil
	}
	return m
}

// SetHeaders stores key value pairs in the returned context that can later
// be retrieved by [HeadersFromContext]. Values stored in this manner will
// automatically be retrieved by client libraries and sent as outgoing headers
// on all requests. keyvals should have a corresponding value for every key
// provided. If there is an odd number of keyvals this method will panic.
func SetHeaders(ctx context.Context, keyvals ...string) context.Context {
	if len(keyvals)%2 != 0 {
		panic(fmt.Sprintf("callctx: an even number of key value pairs must be provided, got %d", len(keyvals)))
	}
	h, ok := ctx.Value(headerKey).(map[string][]string)
	if !ok {
		h = make(map[string][]string)
	}
	for i := 0; i < len(keyvals); i = i + 2 {
		h[keyvals[i]] = append(h[keyvals[i]], keyvals[i+1])
	}
	return context.WithValue(ctx, headerKey, h)
}