From a9daa3c5eb91fc85772dd94da1472cfbd8ba7e13 Mon Sep 17 00:00:00 2001
From: Daniel Lemire <daniel@lemire.me>
Date: Mon, 13 Apr 2026 17:56:53 -0400
Subject: [PATCH] new file

---
 .../generic/ondemand/key_selector_iterator.h  | 131 ++++++++++++++++++
 1 file changed, 131 insertions(+)
 create mode 100644 include/simdjson/generic/ondemand/key_selector_iterator.h

diff --git a/include/simdjson/generic/ondemand/key_selector_iterator.h b/include/simdjson/generic/ondemand/key_selector_iterator.h
new file mode 100644
index 000000000..3a7a58fe7
--- /dev/null
+++ b/include/simdjson/generic/ondemand/key_selector_iterator.h
@@ -0,0 +1,131 @@
+#ifndef SIMDJSON_GENERIC_ONDEMAND_KEY_SELECTOR_ITERATOR_H
+
+#ifndef SIMDJSON_CONDITIONAL_INCLUDE
+#define SIMDJSON_GENERIC_ONDEMAND_KEY_SELECTOR_ITERATOR_H
+#include "simdjson/generic/ondemand/base.h"
+#include "simdjson/generic/ondemand/key_selector.h"
+#include "simdjson/generic/ondemand/object.h"
+#include "simdjson/generic/ondemand/object_iterator.h"
+#include "simdjson/generic/ondemand/field.h"
+#include "simdjson/generic/ondemand/value.h"
+#endif // SIMDJSON_CONDITIONAL_INCLUDE
+
+#include <array>
+#include <cstddef>
+#include <utility>
+
+#if SIMDJSON_SUPPORTS_CONCEPTS
+
+namespace simdjson {
+namespace SIMDJSON_IMPLEMENTATION {
+namespace ondemand {
+
+/**
+ * Forward iterator over selector matches in an object.
+ *
+ * Walks the JSON object once, yielding each (selector_index, value) pair whose
+ * key matches one of the selector's keys, in JSON order. Duplicate matches of
+ * the same key are silently skipped; iteration ends when every selector key
+ * has matched OR the object ends.
+ *
+ *   for (auto [i, val] : obj.select<sel_t>()) {
+ *     switch (i) { case 0: ...; case 1: ...; }
+ *   }
+ */
+template <typename Selector>
+class selector_iterator {
+public:
+    // Yield ondemand::value directly (not simdjson_result<value>); the caller
+    // uses iterator state (error()) to check for errors after iteration.
+    using value_type = std::pair<std::size_t, value>;
+
+    struct end_sentinel {};
+
+    simdjson_inline selector_iterator() noexcept = default;
+
+    simdjson_inline explicit selector_iterator(object obj) noexcept
+      : obj_{std::move(obj)} {
+        auto begin_res = obj_.begin();
+        if (begin_res.error()) { done_ = true; last_error_ = begin_res.error(); return; }
+        it_ = begin_res.value();
+        advance();
+    }
+
+    simdjson_inline value_type operator*() noexcept {
+        return { current_index_, std::move(current_value_) };
+    }
+
+    simdjson_inline selector_iterator& operator++() noexcept { advance(); return *this; }
+
+    simdjson_inline bool operator==(end_sentinel) const noexcept { return done_; }
+    simdjson_inline bool operator!=(end_sentinel) const noexcept { return !done_; }
+
+    /** Error code set if iteration was terminated by an error. */
+    simdjson_inline error_code error() const noexcept { return last_error_; }
+    /** Number of unique selector-key matches produced so far. */
+    simdjson_inline std::size_t matched_count() const noexcept { return matched_; }
+
+private:
+    object obj_{};
+    object_iterator it_{};
+    std::array<bool, Selector::size()> seen_{};
+    std::size_t matched_{0};
+    std::size_t current_index_{Selector::size()};
+    value current_value_{};
+    error_code last_error_{SUCCESS};
+    bool done_{false};
+    bool primed_{false};
+
+    simdjson_inline void advance() noexcept {
+        if (done_) return;
+        if (primed_) { ++it_; primed_ = false; }
+        if (matched_ >= Selector::size()) { done_ = true; return; }
+
+        object_iterator end{};
+        while (it_ != end) {
+            auto f_res = *it_;
+            if (f_res.error()) { last_error_ = f_res.error(); done_ = true; return; }
+            field f = f_res.value_unsafe();
+            std::size_t idx = Selector::match_raw(f.key());
+            if (idx < Selector::size() && !seen_[idx]) {
+                seen_[idx] = true;
+                ++matched_;
+                current_index_ = idx;
+                current_value_ = std::move(f.value());
+                primed_ = true;
+                return;
+            }
+            ++it_;
+        }
+        done_ = true;
+    }
+};
+
+/**
+ * Range adapter returned by object::select<Selector>(). Satisfies the range-for
+ * loop requirements (begin() / end()).
+ */
+template <typename Selector>
+class selector_range {
+public:
+    simdjson_inline explicit selector_range(object obj) noexcept
+      : obj_{std::move(obj)} {}
+
+    simdjson_inline selector_iterator<Selector> begin() noexcept {
+        return selector_iterator<Selector>{std::move(obj_)};
+    }
+    simdjson_inline typename selector_iterator<Selector>::end_sentinel end() const noexcept {
+        return {};
+    }
+
+private:
+    object obj_;
+};
+
+} // namespace ondemand
+} // namespace SIMDJSON_IMPLEMENTATION
+} // namespace simdjson
+
+#endif // SIMDJSON_SUPPORTS_CONCEPTS
+
+#endif // SIMDJSON_GENERIC_ONDEMAND_KEY_SELECTOR_ITERATOR_H