-- context.lua — in-memory conversation history + token budget.
-- Phase 0: ordered turn list, sliding-window eviction by max_turns.
-- Tokenization is char/4 heuristic in Phase 0; accurate count is Phase 3 (Q1).
-- See docs/PHASE0.md §6, §8.

local M = {}

-- The §6 default system prompt. The `CMD: ` (exact prefix, single space)
-- contract is locked per §3 invariants — do not edit without amending PHASE0.
local DEFAULT_SYSTEM_PROMPT = [[
You are aish, an AI-augmented shell assistant. You help the user execute shell
commands, write and debug code, and re-engineer software. When suggesting shell
commands, output them on a line beginning with exactly "CMD: " so aish can
identify and optionally execute them. Be concise. Prefer concrete actions over
explanations unless asked.]]

local Context = {}
Context.__index = Context

function M.new(opts)
    opts = opts or {}
    return setmetatable({
        system_prompt = opts.system_prompt or DEFAULT_SYSTEM_PROMPT,
        turns         = {},
        max_turns     = opts.max_turns    or 40,
        token_budget  = opts.token_budget or 4096,
    }, Context)
end

function Context:append(turn)
    assert(type(turn) == "table" and turn.role and turn.content,
           "context:append requires { role = ..., content = ... }")
    self.turns[#self.turns + 1] = { role = turn.role, content = turn.content }
end

-- Render the messages array for broker.chat (system prompt prepended; turns
-- in order). The system prompt is NOT stored in self.turns per §6.
function Context:to_messages()
    local msgs = { { role = "system", content = self.system_prompt } }
    for _, t in ipairs(self.turns) do
        msgs[#msgs + 1] = { role = t.role, content = t.content }
    end
    return msgs
end

-- Evict the oldest pair (user + assistant) while we exceed max_turns. Returns
-- total turns evicted. Caller is responsible for rendering the §8 status line.
function Context:enforce_budget()
    local evicted = 0
    while #self.turns > self.max_turns do
        table.remove(self.turns, 1)
        evicted = evicted + 1
        if #self.turns > self.max_turns or evicted % 2 == 1 then
            if #self.turns > 0 then
                table.remove(self.turns, 1)
                evicted = evicted + 1
            end
        end
    end
    return evicted
end

-- Coarse char/4 token estimate per §8. Phase 0 visibility only; accurate
-- tokenization is Q1 (target Phase 3).
function Context:estimate_tokens()
    local n = #self.system_prompt
    for _, t in ipairs(self.turns) do
        n = n + #t.content
    end
    return math.floor(n / 4)
end

function Context:reset()
    self.turns = {}
end

return M