Question

How to find the longest shared prefix among all words in a string with Swift

Answer 1

import Foundation

// Extract alphabetic words (lowercased)
func extractWords(_ text: String) -> [String] {
    let regex = try! NSRegularExpression(pattern: "[A-Za-z]+")
    let range = NSRange(text.startIndex..., in: text)

    return regex.matches(in: text, range: range).compactMap {
        Range($0.range, in: text).map { String(text[$0]).lowercased() }
    }
}

// Build prefix → list of words that share that prefix
func groupByPrefixes(_ words: [String]) -> [String: [String]] {
    var groups: [String: [String]] = [:]

    for w in words {
        for i in 1...w.count {
            let prefix = String(w.prefix(i))
            groups[prefix, default: []].append(w)
        }
    }

    return groups
}

// Find the longest prefix that appears in 2+ words
func longestSharedPrefix(_ groups: [String: [String]]) -> String? {
    groups
        .filter { $0.value.count >= 2 }
        .keys
        .max(by: { $0.count < $1.count })
}

// ------------------------------------------------------------
// Main
// ------------------------------------------------------------

let text = """
The Lowly inhabitants of the lowland were surprised to see
the lower branches of the trees.
"""

let words = extractWords(text)
let groups = groupByPrefixes(words)
let prefix = longestSharedPrefix(groups)

if let p = prefix, let group = groups[p] {
    print("Longest shared prefix:")
    print("\(p) | prefix_len=\(p.count) | group_count=\(group.count) | \(group)")
} else {
    print("No shared prefix found.")
}



/*
run:

Longest shared prefix:
lowl | prefix_len=4 | group_count=2 | ["lowly", "lowland"]

*/

 

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