nvim/lua/git/diff.lua

local log = require("log")
local repo = require("git.repo")
local util = require("util")

local M = {}

---@param buf integer
---@param worktree string
---@param path string
local function attach_index_writer(buf, worktree, path)
    vim.api.nvim_create_autocmd("BufWriteCmd", {
        buffer = buf,
        callback = function()
            local body = table.concat(
                vim.api.nvim_buf_get_lines(buf, 0, -1, false),
                "\n"
            ) .. "\n"
            local hash_stdout = util.system_sync(
                { "git", "hash-object", "-w", "--stdin" },
                { cwd = worktree, stdin = body }
            )
            if not hash_stdout then
                return
            end
            local sha = vim.trim(hash_stdout)
            local mode = vim.b[buf].git_index_mode
            if not mode then
                mode = "100644"
                local ls = util.system_sync(
                    { "git", "ls-files", "-s", "--", path },
                    { cwd = worktree, silent = true }
                )
                if ls then
                    local m = ls:match("^(%d+)")
                    if m then
                        mode = m
                    end
                end
                vim.b[buf].git_index_mode = mode
            end
            -- Use the 3-arg form (mode sha path) instead of the comma form
            -- (mode,sha,path), which doesn't survive paths containing a
            -- comma.
            if
                not util.system_sync({
                    "git",
                    "update-index",
                    "--cacheinfo",
                    mode,
                    sha,
                    path,
                }, { cwd = worktree })
            then
                return
            end
            vim.bo[buf].modified = false
        end,
    })
end

---Return a buffer holding the content at `<ref>:<path>`. `ref` is
---"index", "HEAD", or a commit revspec.
---@param worktree string
---@param ref string
---@param path string
---@return integer
function M.git_show_buf(worktree, ref, path)
    local name = "git://" .. ref .. "//" .. path
    local buf = vim.fn.bufadd(name)
    vim.b[buf].git_worktree = worktree
    vim.fn.bufload(buf)
    return buf
end

---BufReadCmd handler for `git://<ref>//<path>` URIs. Worktree comes from
---`vim.b[buf].git_worktree` if set, else from cwd. Ref of "index" maps
---to `git show :<path>`; "worktree" leaves the buffer empty (placeholder
---for missing files); anything else is a revspec.
---@param buf integer
function M.read_uri(buf)
    local name = vim.api.nvim_buf_get_name(buf)
    local ref, path = name:match("^git://(.-)//(.*)$")
    if not ref or path == "" then
        return
    end
    ---@cast path -nil

    local worktree = vim.b[buf].git_worktree or select(2, repo.resolve_cwd())
    if not worktree then
        log.error("git BufReadCmd %s: cannot resolve worktree", name)
        return
    end
    vim.b[buf].git_worktree = worktree

    vim.bo[buf].swapfile = false
    vim.bo[buf].bufhidden = "wipe"

    if ref == "worktree" then
        vim.bo[buf].buftype = "nofile"
        vim.bo[buf].modifiable = false
        vim.bo[buf].modified = false
        vim.api.nvim_exec_autocmds("BufReadPost", { buffer = buf })
        return
    end

    local revspec = ref == "index" and (":" .. path) or (ref .. ":" .. path)
    local stdout = util.system_sync(
        { "git", "show", revspec },
        { cwd = worktree }
    )
    if stdout then
        vim.api.nvim_buf_set_lines(buf, 0, -1, false, util.split_lines(stdout))
    end

    if ref == "index" then
        vim.bo[buf].buftype = "acwrite"
        -- Re-running BufReadCmd (e.g. on `:edit`) would otherwise stack
        -- another BufWriteCmd on the same buffer, so each `:w` runs
        -- hash-object + update-index N times.
        if not vim.b[buf].git_index_writer then
            attach_index_writer(buf, worktree, path)
            vim.b[buf].git_index_writer = true
        end
    else
        vim.bo[buf].buftype = "nofile"
        vim.bo[buf].modifiable = false
    end
    vim.bo[buf].modified = false

    -- BufReadCmd suppresses the normal BufReadPost dispatch, so filetype
    -- detection and modeline parsing don't run unless we fire it ourselves.
    vim.api.nvim_exec_autocmds("BufReadPost", { buffer = buf })
end

---@class ow.Git.EmptyBufOpts
---@field name string?
---@field bufhidden ("hide"|"wipe")?  defaults to "wipe"

---Build a read-only scratch buffer, optionally naming it. When `opts.name`
---is set and a loaded buffer with that name already exists, returns it
---instead of creating a duplicate.
---@param opts ow.Git.EmptyBufOpts?
---@return integer
function M.empty_buf(opts)
    opts = opts or {}
    if opts.name then
        local existing = vim.fn.bufnr(opts.name)
        if existing ~= -1 and vim.api.nvim_buf_is_loaded(existing) then
            return existing
        end
    end
    local buf = vim.api.nvim_create_buf(false, true)
    vim.bo[buf].buftype = "nofile"
    vim.bo[buf].bufhidden = opts.bufhidden or "wipe"
    vim.bo[buf].swapfile = false
    vim.bo[buf].modifiable = false
    vim.bo[buf].modified = false
    if opts.name then
        pcall(vim.api.nvim_buf_set_name, buf, opts.name)
    end
    return buf
end

---@param abs_path string
---@return integer
function M.load_file_buf(abs_path)
    local buf = vim.fn.bufadd(abs_path)
    vim.fn.bufload(buf)
    return buf
end

---Name a scratch buffer with a `git://...` URI and apply the filetype
---inferred from the inner path segment. The `nvim_buf_set_name` call is
---wrapped in pcall because a buffer with that name may already exist
---(E95).
---@param buf integer
---@param name string
function M.set_buf_name_and_filetype(buf, name)
    pcall(vim.api.nvim_buf_set_name, buf, name)
    local ft = vim.filetype.match({ buf = buf })
    if ft then
        vim.bo[buf].filetype = ft
    end
end

---Toggle a window into or out of Vim's diff mode. Goes through the
---`:diffthis` / `:diffoff` commands rather than `vim.wo[win].diff = X`.
---The command path runs Vim's full `diff_win_options` setup, which sets
---an internal flag that prevents subsequently-created floats from
---inheriting `'diff' = 1` when opened from a focused diff pane. The raw
---option setter skips that setup, so floats (oil, fzf-lua, etc) end up
---joining the tabpage's diff group and corrupting its render.
---@param win integer
---@param enabled boolean
function M.set_diff(win, enabled)
    vim.api.nvim_win_call(win, function()
        vim.cmd(enabled and "diffthis" or "diffoff")
    end)
end

---@class ow.Git.SplitOpts
---@field ref string '' for index, 'HEAD' for HEAD
---@field vertical boolean

---@param opts ow.Git.SplitOpts
function M.split(opts)
    local cur_buf = vim.api.nvim_get_current_buf()
    local cur_path = vim.api.nvim_buf_get_name(cur_buf)
    if cur_path == "" then
        log.warning("no file in current buffer")
        return
    end
    if vim.bo[cur_buf].buftype ~= "" then
        log.warning("cannot diff this buffer (not a worktree file)")
        return
    end
    local _, worktree = repo.resolve(cur_path)
    if not worktree then
        log.warning("not in a git repository")
        return
    end
    local rel = vim.fs.relpath(worktree, cur_path)
    if not rel then
        log.warning("file is outside the worktree")
        return
    end

    local label = opts.ref == "" and "index" or opts.ref
    local uri = "git://" .. label .. "//" .. rel
    -- Stash the worktree on the buffer so the BufReadCmd handler doesn't
    -- fall back to cwd resolution (wrong when cwd != worktree).
    local buf = vim.fn.bufadd(uri)
    vim.b[buf].git_worktree = worktree

    local prefix = opts.vertical and "leftabove vert " or "leftabove "
    vim.cmd(prefix .. "diffsplit " .. vim.fn.fnameescape(uri))
end

return M