Calculate a preferred context size for ollama calls (num_ctx).

calcContextSize.ts

const MIN_CONTEXT_SIZE = 4096
const MAX_CONTEXT_SIZE = MIN_CONTEXT_SIZE * 8

/**
 * Calculates the context size for a given set of messages, generically
 * in tokens.
 *
 * @param messages - The messages to calculate the context size for.
 * @param maxContextSize - The maximum context size to return. Defaults to MAX_CONTEXT_SIZE.
 * @returns The context size in tokens.
 */
export function calcContextSize(
  messages: PromptMessage[],
  maxContextSize: number = MAX_CONTEXT_SIZE
) {
  const charLength = messages.reduce((acc, msg) => acc + msg.content.length, 0)
  const tokenLength = (charLength / 4) * 2
  const contextSize = Math.min(
    Math.max(tokenLength, MIN_CONTEXT_SIZE),
    maxContextSize
  )
  const preferredContextSize = Math.ceil(contextSize / 1024) * 1024
  return preferredContextSize
}

Where PromptMessage is:

type PromptMessage = {
  role: PromptRole
  content: string
  images?: Uint8Array[] | string[] // Base64 encoded images
}