What is a GitLab line_code as referenced when creating a new merge request thread

Question

I'm trying to create a discussion note on a merge request on a certain line of a file with the GitLab api using this endpoint: https://docs.gitlab.com/ee/api/discussions.html#create-new-merge-request-thread

Part of the payload asks for a line_code

Attribute	Type	Required	Description
position[line_range][start][line_code]	string	yes	Line code for the start line

When I issue a POST I get a response with:

    "message": "400 (Bad request) \"Note {:line_code=>[\"can't be blank\", \"must be a valid line code\"], :position=>[\"is incomplete\"]}\" not given"

What is this line_code? Is it some kind of calculated value? The documentation is rather vague here.

When I issue a GET for all the current notes on a merge_request I can see some notes have this line_code (see below). I'm trying to figure out how to create that value for new notes.

 {
        "id": 89,
        "type": "DiffNote",
        "body": "4",
        "attachment": null,
        "author": {
            "id": 6,
            "name": "brian c",
            "username": "bc",
            "state": "active",
            "avatar_url": "https://www.gravatar.com/avatar/f590a9cf57136732dd0cb5z9b1563390?s=80&d=identicon",
            "web_url": "http://gitlab.mycompany.us/thisIsMe"
        },
        "created_at": "2021-01-11T21:46:23.861Z",
        "updated_at": "2021-01-11T21:46:23.861Z",
        "system": false,
        "noteable_id": 21,
        "noteable_type": "MergeRequest",
        "position": {
            "base_sha": "3bf8094f0d54fc70a66698bd582f25c77243de3b",
            "start_sha": "3bf8094f0d54fc70a66698bd582f25c77243de3b",
            "head_sha": "a10e73cf84eae38286df56f4b58fa221d7eefc44",
            "old_path": "b.txt",
            "new_path": "b.txt",
            "position_type": "text",
            "old_line": null,
            "new_line": 4,
            "line_range": {
                "start": {
                    "line_code": "aceba96ffdf13ce4cd4171c0248420cc03108ef0_0_4",
                    "type": "new",
                    "old_line": null,
                    "new_line": 4
                },
                "end": {
                    "line_code": "aceba96ffdf13ce4cd4171c0248420cc03108ef0_0_4",
                    "type": "new",
                    "old_line": null,
                    "new_line": 4
                }
            }
        },
        "resolvable": true,
        "resolved": false,
        "resolved_by": null,
        "confidential": false,
        "noteable_iid": 3,
        "commands_changes": {}
    },

Answer 1

Line code is hash of the file name + underscore + old line number + underscore + new line number

The documentation is wrong. line_code is required only if you are using position.line_range which is only required for adding diff note spanning multiple lines of diff. You don't need to deal with line_code for single-line diff notes. So it is not a required parameter. You can just use position.old_line or position.new_line.