anko / slice-with-context

JS module: slice string, but with context (useful for showing parse errors with context)

slice-with-context 

Basically String.prototype.slice, but with a configurably-positioned window around it, showing context.

npm i slice-with-context

The module exports 1 function:

sliceWithContext(inputString, windowSize, cutOffset, cutLength [, windowLeftBias] [, overflowLeftBias])

Params:

• inputString:String string to cut

• windowSize:Number size of window

• cutOffset:Number cut start position, in characters

• cutLength:Number cut length, in characters

• windowLeftBias:Number between 0–1 [optional] if there is space left in the window on both sides of the sliced part, this option determines what proportion of what's left to show on the left rather than the right.

  0 means only show context on the left. 1 means only show context on the right. (default 0.5)

• overflowLeftBias:Numberbetween 0–1 [optional] if the window is too small to show the sliced part fully, this option determines which part of the sliced part should be shown.

  0 means only show the very start of the slice. 1 means only show the very end of the slice. Intermediate values mean to show that far along the middle of the slice. (default 0)

Throws if the slice parameters would miss the string entirely, by being too long, offset past it, or other such obviously wrong nonsense.

Clamps windowLeftBias and overflowLeftBias to 0–1.

Example

Suppose you want to slice the bold part out of 123456789, showing a total of 6 characters; the sliced part and the rest just for context because you have space to spare.

const sliceWithContext = require('slice-with-context')
const output = sliceWithContext(
  '1234567890',
  6,    // windowSize
  4,    // offset
  4,    // length
  0.5, // windowLeftBias
  0,    // overflowLeftBias
)
console.log(output)

{
  fit: true,
  withContext: '456789',
  visibleSlice: '5678',
  fullSlice: '5678',
  position: { offset: 1, length: 4 }
}

If you're writing a compiler or something, you might render that as—

456789
 ^^^^

Or use ANSI colour codes, or some such.

Specific situations

Equal amounts of context on either side

const sliceWithContext = require('slice-with-context')
const output = sliceWithContext(
  '1234567890',
  6,   // windowSize
  4,   // offset
  4,   // length
  0.5, // windowLeftBias
  0,   // overflowLeftBias
)
console.log(output)

{
  fit: true,
  withContext: '456789',
  visibleSlice: '5678',
  fullSlice: '5678',
  position: { offset: 1, length: 4 }
}

With context only on the left

const sliceWithContext = require('slice-with-context')
const output = sliceWithContext(
  '1234567890',
  6, // windowSize
  4, // offset
  4, // length
  1, // windowLeftBias
  0, // overflowLeftBias
)
console.log(output)

{
  fit: true,
  withContext: '345678',
  visibleSlice: '5678',
  fullSlice: '5678',
  position: { offset: 2, length: 4 }
}

With context only on the right

const sliceWithContext = require('slice-with-context')
const output = sliceWithContext(
  '1234567890',
  6, // windowSize
  4, // offset
  4, // length
  0, // windowLeftBias
  0, // overflowLeftBias
)
console.log(output)

{
  fit: true,
  withContext: '567890',
  visibleSlice: '5678',
  fullSlice: '5678',
  position: { offset: 0, length: 4 }
}

Left context only, but with no space

const sliceWithContext = require('slice-with-context')
const output = sliceWithContext(
  '1234567890',
  6, // windowSize
  0, // offset
  4, // length
  1, // windowLeftBias
  0, // overflowLeftBias
)
console.log(output)

{
  fit: true,
  withContext: '123456',
  visibleSlice: '1234',
  fullSlice: '1234',
  position: { offset: 0, length: 4 }
}

Both-sided context, but no space at end

const sliceWithContext = require('slice-with-context')
const output = sliceWithContext(
  '1234567890',
  6, // windowSize
  6, // offset
  4, // length
  0.5, // windowLeftBias
  0, // overflowLeftBias
)
console.log(output)

{
  fit: true,
  withContext: '567890',
  visibleSlice: '7890',
  fullSlice: '7890',
  position: { offset: 2, length: 4 }
}

Insufficient space in window, truncated to show start

const sliceWithContext = require('slice-with-context')
const output = sliceWithContext(
  '1234567890',
  6, // windowSize
  0, // offset
  8, // length
  1, // windowLeftBias
  1, // overflowLeftBias
)
console.log(output)

{
  fit: false,
  withContext: '123456',
  visibleSlice: '123456',
  fullSlice: '12345678',
  position: { offset: 0, length: 6 }
}

Insufficient space in window, truncated to show end

const sliceWithContext = require('slice-with-context')
const output = sliceWithContext(
  '1234567890',
  6, // windowSize
  0, // offset
  8, // length
  1, // windowLeftBias
  0, // overflowLeftBias
)
console.log(output)

{
  fit: false,
  withContext: '345678',
  visibleSlice: '345678',
  fullSlice: '12345678',
  position: { offset: 0, length: 6 }
}

Tests

That's the code examples above! They're run with txm.