Function FirstIndexInBudget

pkg/compaction/compaction.go:109–123

FirstIndexInBudget returns the smallest index N such that messages[N:] fits within contextLimit, snapping to a user/assistant turn boundary. Used to truncate the conversation handed to the summarization model so the request itself doesn't blow the context window. When the entire slice fits within c

(messages []chat.Message, contextLimit int64)

// no user/asst turns, it returns len(messages); callers should treat
// that as "nothing to send" and skip the truncation.
func FirstIndexInBudget(messages []chat.Message, contextLimit int64) int {
	var tokens int64
	lastValidMessageSeen := len(messages)
	for i := range slices.Backward(messages) {
		tokens += EstimateMessageTokens(&messages[i])
		if tokens > contextLimit {
			return lastValidMessageSeen
		}
		role := messages[i].Role
		if role == chat.MessageRoleUser || role == chat.MessageRoleAssistant {
			lastValidMessageSeen = i
		}
	}
	return lastValidMessageSeen
}