Douglas McIlroy, the original author of the Unix spell command, responded to an article detailing its inner workings with further insights into its development. He clarified that the efficient hashing used wasn't a conscious optimization but rather a side effect of the limited memory available on the PDP-7. The stop word list was chosen pragmatically to shrink the dictionary size. McIlroy also revealed that he experimented with stemming algorithms, ultimately discarding them due to excessive performance overhead and concerns about false positives. He highlighted the importance of spell's collaborative development, with Steve Johnson's later refinements significantly improving its accuracy and efficiency.
A Twitter post by Abhijeet Rastogi (@abhi9u) highlights a recent communication from Douglas McIlroy, one of the original Unix developers, regarding the historical implementation of the Unix spell command. McIlroy's response provides further clarification and previously undisclosed details about the ingenious techniques employed in the early versions of this utility. The original spell command, known for its efficiency despite limited computing resources, leveraged a clever combination of hashing and a pre-generated list of correctly spelled words.
McIlroy elaborates on the specific hashing algorithm used, a variant involving the exclusive-or (XOR) operation applied iteratively to character codes within each input word. This XOR-based hashing function, unlike simpler hashing methods, incorporated a rotational element to distribute hashed values more evenly. The resulting hash values were then used to index into a bit table, effectively representing a compact set of correctly spelled words.
Critically, McIlroy's communication reveals that the original implementation actually used two such hash tables. This two-table approach significantly reduced the incidence of false positives, where incorrectly spelled words might accidentally produce the same hash value as a correctly spelled word. By requiring a match in both tables, the spell command achieved a higher degree of accuracy.
Furthermore, McIlroy's response sheds light on the method used to generate the word list itself. He explains that they derived the word list from the Webster's Second International Dictionary, specifically by processing the phototypesetter tapes used in its printing. This method allowed them to efficiently extract a comprehensive vocabulary for the spell command's use.
In conclusion, the Twitter post showcases McIlroy's valuable insights into the early development of the Unix spell command, unveiling the specific XOR-based hashing algorithm, the dual-table approach for enhanced accuracy, and the source of the word list itself. This information adds historical context and technical depth to our understanding of this fundamental Unix utility and the innovative solutions employed in its creation.
Summary of Comments ( 1 )
https://news.ycombinator.com/item?id=42962394
HN commenters discuss McIlroy's response regarding the original Unix spell program. Several express fascination with the historical context and McIlroy's continued engagement with the topic. Some highlight the elegance and efficiency of the original implementation, particularly its use of hashing and minimal resources. Others note the contrast between then-current hardware limitations and modern capabilities, marveling at what was achieved with so little. A few commenters delve into specific technical details, such as the choice of hashing algorithms and the use of a 64KB PDP-11. The overall sentiment is one of appreciation for both McIlroy's contribution and the ingenuity of early Unix development.
The Hacker News post titled "Douglas McIlroy responds to Unix spell article with new implementation details" links to a Twitter thread where McIlroy provides further insights into the development of the Unix spell program. The discussion on Hacker News is relatively brief, containing only a handful of comments, and doesn't delve deep into the technical details. It predominantly focuses on McIlroy's historical contributions and the ingenuity of early Unix utilities.
One commenter expresses admiration for McIlroy's continuing engagement with the history and evolution of these tools, noting the privilege of receiving such insights directly from the source. They highlight the elegance and efficiency of the original spell implementation, emphasizing its lasting impact.
Another comment briefly touches on the historical context, mentioning Ken Thompson's role and the development environment at the time. It underscores the limited resources available then, contributing to the impressive nature of the achievements.
The remaining comments are concise, expressing appreciation for the historical details shared and acknowledging McIlroy's significant contribution to Unix. One commenter specifically points out the value of learning about the original design constraints and decisions.
The discussion lacks substantial technical analysis of McIlroy's new information. It serves more as a space for acknowledging his contributions and expressing appreciation for the historical context he provides. There's no significant debate or contrasting viewpoints presented in the limited number of comments.