Punycode is an algorithm that encodes Unicode strings into ASCII for compatibility with DNS, which only supports ASCII characters. Ian Duncan implemented a Haskell version of Punycode for his idn package while working on a JSON Schema library. He discovered that Punycode is more complex than it appears, cleverly navigating the constraints of encoding by using techniques like adaptive bias adjustment, variable-length encoding, and delta compression. This efficiency is particularly important since most modern systems handle UTF-8 without considering the encoding intricacies that Punycode must address. The algorithm processes text by first outputting all ASCII characters as-is, then adding a delimiter before encoding non-ASCII characters based on their code points. It measures the "jump" between code points and uses variable-length base-36 encoding for these deltas, allowing small deltas to use fewer digits. A significant feature is its adaptive encoding, which adjusts thresholds based on the specific text being encoded. This allows Punycode to handle different languages effectively, optimizing for the context of the characters. Sorting characters by code points rather than their order in the input string means that duplicate characters are processed consecutively, resulting in zero deltas for those duplicates. This approach takes advantage of the commonality in human writing, where consecutive characters often belong to the same script. Duncan's article attempts to unpack the design choices behind the Punycode RFC, which lacks detailed explanations for its decisions, providing insights based on his implementation experience.