feat(chat): support function call api (#369)

* feat(chat): support function call api

* rename struct & add const ChatMessageRoleFunction

chat.go

@@ -11,8 +11,11 @@ const (
 	ChatMessageRoleSystem    = "system"
 	ChatMessageRoleUser      = "user"
 	ChatMessageRoleAssistant = "assistant"
+	ChatMessageRoleFunction  = "function"
 )
 
+const chatCompletionsSuffix = "/chat/completions"
+
 var (
 	ErrChatCompletionInvalidModel       = errors.New("this model is not supported with this method, please use CreateCompletion client method instead") //nolint:lll
 	ErrChatCompletionStreamNotSupported = errors.New("streaming is not supported with this method, please use CreateChatCompletionStream")              //nolint:lll
@@ -27,6 +30,14 @@ type ChatCompletionMessage struct {
 	// - https://github.com/openai/openai-python/blob/main/chatml.md
 	// - https://github.com/openai/openai-cookbook/blob/main/examples/How_to_count_tokens_with_tiktoken.ipynb
 	Name string `json:"name,omitempty"`
+
+	FunctionCall *FunctionCall `json:"function_call,omitempty"`
+}
+
+type FunctionCall struct {
+	Name string `json:"name,omitempty"`
+	// call function with arguments in JSON format
+	Arguments string `json:"arguments,omitempty"`
 }
 
 // ChatCompletionRequest represents a request structure for chat completion API.
@@ -43,12 +54,70 @@ type ChatCompletionRequest struct {
 	FrequencyPenalty float32                 `json:"frequency_penalty,omitempty"`
 	LogitBias        map[string]int          `json:"logit_bias,omitempty"`
 	User             string                  `json:"user,omitempty"`
+	Functions        []*FunctionDefine       `json:"functions,omitempty"`
+	FunctionCall     string                  `json:"function_call,omitempty"`
 }
 
+type FunctionDefine struct {
+	Name        string `json:"name"`
+	Description string `json:"description,omitempty"`
+	// it's required in function call
+	Parameters *FunctionParams `json:"parameters"`
+}
+
+type FunctionParams struct {
+	// the Type must be JSONSchemaTypeObject
+	Type       JSONSchemaType               `json:"type"`
+	Properties map[string]*JSONSchemaDefine `json:"properties,omitempty"`
+	Required   []string                     `json:"required,omitempty"`
+}
+
+type JSONSchemaType string
+
+const (
+	JSONSchemaTypeObject  JSONSchemaType = "object"
+	JSONSchemaTypeNumber  JSONSchemaType = "number"
+	JSONSchemaTypeString  JSONSchemaType = "string"
+	JSONSchemaTypeArray   JSONSchemaType = "array"
+	JSONSchemaTypeNull    JSONSchemaType = "null"
+	JSONSchemaTypeBoolean JSONSchemaType = "boolean"
+)
+
+// JSONSchemaDefine is a struct for JSON Schema.
+type JSONSchemaDefine struct {
+	// Type is a type of JSON Schema.
+	Type JSONSchemaType `json:"type,omitempty"`
+	// Description is a description of JSON Schema.
+	Description string `json:"description,omitempty"`
+	// Enum is a enum of JSON Schema. It used if Type is JSONSchemaTypeString.
+	Enum []string `json:"enum,omitempty"`
+	// Properties is a properties of JSON Schema. It used if Type is JSONSchemaTypeObject.
+	Properties map[string]*JSONSchemaDefine `json:"properties,omitempty"`
+	// Required is a required of JSON Schema. It used if Type is JSONSchemaTypeObject.
+	Required []string `json:"required,omitempty"`
+}
+
+type FinishReason string
+
+const (
+	FinishReasonStop          FinishReason = "stop"
+	FinishReasonLength        FinishReason = "length"
+	FinishReasonFunctionCall  FinishReason = "function_call"
+	FinishReasonContentFilter FinishReason = "content_filter"
+	FinishReasonNull          FinishReason = "null"
+)
+
 type ChatCompletionChoice struct {
-	Index        int                   `json:"index"`
-	Message      ChatCompletionMessage `json:"message"`
-	FinishReason string                `json:"finish_reason"`
+	Index   int                   `json:"index"`
+	Message ChatCompletionMessage `json:"message"`
+	// FinishReason
+	// stop: API returned complete message,
+	// or a message terminated by one of the stop sequences provided via the stop parameter
+	// length: Incomplete model output due to max_tokens parameter or token limit
+	// function_call: The model decided to call a function
+	// content_filter: Omitted content due to a flag from our content filters
+	// null: API response still in progress or incomplete
+	FinishReason FinishReason `json:"finish_reason"`
 }
 
 // ChatCompletionResponse represents a response structure for chat completion API.
@@ -71,7 +140,7 @@ func (c *Client) CreateChatCompletion(
 		return
 	}
 
-	urlSuffix := "/chat/completions"
+	urlSuffix := chatCompletionsSuffix
 	if !checkEndpointSupportsModel(urlSuffix, request.Model) {
 		err = ErrChatCompletionInvalidModel
 		return