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<meta http-equiv="X-UA-Compatible" content="IE=edge"><title>Enum Stringer Interface optimisation in Go - hakaselogs</title><link rel="icon" type="image/png" href=/images/favicon.png /><meta name="viewport" content="width=device-width, initial-scale=1">
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<span class="day">08</span>
<span class="rest">Apr 2021</span>
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<h1 class="title">Enum Stringer Interface optimisation in Go</h1>
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<p>The most idiomatic way of describing an enum type in Go is to use constants, often in conjunction with <a href="https://golang.org/ref/spec#Iota">iota</a>.</p>
<p>A pattern I’ve used to implement the <code>Stringer</code> interface for an enum type is to
lookup the string representation in a slice. Consider this snippet from a
project I’m working on:</p>
<div class="highlight"><pre style="background-color:#fff;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-go" data-lang="go"><span style="color:#008000">// Status ...
</span><span style="color:#008000"></span><span style="color:#00f">type</span> Status <span style="color:#2b91af">uint32</span>
<span style="color:#008000">// List of possible status values
</span><span style="color:#008000"></span><span style="color:#00f">const</span> (
<span style="color:#008000">// The operation is known, but hasn't been decided yet
</span><span style="color:#008000"></span> Processing Status = <span style="color:#00f">iota</span>
<span style="color:#008000">// The operation will never be accepted
</span><span style="color:#008000"></span> Rejected
<span style="color:#008000">// The operation was accepted
</span><span style="color:#008000"></span> Accepted
...
)
<span style="color:#00f">func</span> (s Status) String() <span style="color:#2b91af">string</span> {
<span style="color:#00f">return</span> []<span style="color:#2b91af">string</span>{<span style="color:#a31515">"processing"</span>, <span style="color:#a31515">"rejected"</span>, <span style="color:#a31515">"accepted"</span>}[s]
}
</code></pre></div><p>The snippet above works correct but has a few glitches – passing
<code>Status(-2)</code> or <code>Status(300)</code> will cause the <code>String()</code> method to panic, and
when appending to the slice, the order of the enum constants has to be taken
into consideration.</p>
<p>A way to optimise this is to use a map which provides <code>O(1)</code> access time rather
than the linear <code>O(n)</code> time in the string slice version.</p>
<div class="highlight"><pre style="background-color:#fff;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-go" data-lang="go">...
<span style="color:#008000">// Map of Status values back to their constant names for pretty printing.
</span><span style="color:#008000"></span><span style="color:#00f">var</span> txStatuses = <span style="color:#00f">map</span>[Status]<span style="color:#2b91af">string</span>{
Processing: <span style="color:#a31515">"processing"</span>,
Rejected: <span style="color:#a31515">"rejected"</span>,
Accepted: <span style="color:#a31515">"accepted"</span>,
}
<span style="color:#008000">// String returns the ErrorCode as a human-readable name.
</span><span style="color:#008000"></span><span style="color:#00f">func</span> (s Status) String() <span style="color:#2b91af">string</span> {
<span style="color:#00f">if</span> s := txStatuses[s]; s != <span style="color:#a31515">""</span> {
<span style="color:#00f">return</span> s
}
<span style="color:#00f">return</span> fmt.Sprintf(<span style="color:#a31515">"Unknown Status (%d)"</span>, uint32(s))
}
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