function getSubtleFunction()

in modules/material-management-browser/src/material_helpers.ts [236:280]
40 lines of code
7 McCabe index (conditional complexity)

    function getSubtleFunction(iv: Uint8Array, additionalData: Uint8Array) {
      /* Precondition: The length of the IV must match the WebCryptoAlgorithmSuite specification. */
      needs(
        iv.byteLength === ivLength,
        'Iv length does not match algorithm suite specification'
      )
      return async (data: Uint8Array) => {
        if (isCryptoKey(deriveKey) && isFullSupportWebCryptoBackend(backend)) {
          const { subtle } = backend
          const algorithm = { name: cipherName, iv, additionalData, tagLength }
          return subtle[subtleFunctionName](algorithm, deriveKey, data)
        } else if (
          !isCryptoKey(deriveKey) &&
          !isFullSupportWebCryptoBackend(backend)
        ) {
          const { nonZeroByteSubtle, zeroByteSubtle } = backend
          const { nonZeroByteCryptoKey, zeroByteCryptoKey } = deriveKey
          const algorithm = { name: cipherName, iv, additionalData, tagLength }
          /* Precondition: The WebCrypto AES-GCM decrypt API expects the data *and* tag together.
           * This means that on decrypt any amount of data less than tagLength is invalid.
           * This also means that zero encrypted data will be equal to tagLength.
           */
          const dataByteLength =
            subtleFunctionName === 'decrypt'
              ? data.byteLength - tagLength / 8
              : data.byteLength
          needs(dataByteLength >= 0, 'Invalid data length.')
          if (dataByteLength === 0) {
            return zeroByteSubtle[subtleFunctionName](
              algorithm,
              zeroByteCryptoKey,
              data
            )
          } else {
            return nonZeroByteSubtle[subtleFunctionName](
              algorithm,
              nonZeroByteCryptoKey,
              data
            )
          }
        }
        // This should be impossible
        throw new Error('Unknown Error')
      }
    }