How TF2 health regen works, in detail

CTFPlayer_RegenThink.cpp

// Health regen stuff
// Based on server_srv.so version 20151007a
// Reverse engineering by sigsegv


/* this Think function is called once per second */
void CTFPlayer::RegenThink()
{
	if (!this->IsAlive())
	{
		// The member variable m_flRegenAmount is stored between calls
		// This means that fractional health regen amounts will build up over time
		// (e.g. 0.2 health regen will result in +1 hp every 5 seconds or so)
		
		if (this->IsPlayerClass(TF_CLASS_MEDIC))
		{
			float flTimeSinceDamage = this->GetLastDamageTime() - gpGlobals->curtime;
			
			// Set the innate medic health regen based on time-since-last-damage:
			// < 5  seconds: +3.0
			// 5-10 seconds: smoothly scales between +3.0 and +6.0
			// > 10 seconds: +6.0
			float flMedicRegen = RemapValClamped(flTimeSinceDamage, 5.0f, 10.0f, 3.0f, 6.0f);
			
			// Handling for the Healing Mastery upgrade
			if (TFGameRules()->GameModeUsesUpgrades())
			{
				int iAttr_HealingMastery = 0;
				CALL_ATTRIB_HOOK_INT(iAttr_HealingMastery, healing_mastery);
				
				if (iAttr_HealingMastery != 0)
				{
					// The attribute value range (0 to 4) is mapped to a scale factor range (1.00x to 2.00x)
					// [ 0: 1.00x    1: 1.25x    2: 1.50x    3: 1.75x    4: 2.00x ]
					float flScaleFactor = RemapValClamped((float)iAttr_HealingMastery, 0.0f, 4.0f, 1.00f, 2.00f);
					flMedicRegen *= flScaleFactor;
				}
			}
			
			this->m_flRegenAmount += flMedicRegen;
		}
		
		// Add/subtract the health regen attribute(s) active on the player
		float flAttr_HealthRegen = 0.0f;
		CALL_ATTRIB_HOOK_FLOAT(flAttr_HealthRegen, add_health_regen);
		this->m_flRegenAmount += flAttr_HealthRegen;
		
		// SKIPPED: Mannpower rune stuff
		
		int iRegenAmount = 0;
		
		// If we are regenning by at least 1 positive health point, then heal
		if (this->m_flRegenAmount >= 1.0f)
		{
			// Get just the integer part of the regen amount
			iRegenAmount = (int)floor(this->m_flRegenAmount);
			
			// Don't bother healing if already at full health
			// This is completely redundant because CBaseEntity::TakeHealth does the exact same check
			if (this->GetHealth() < this->GetMaxHealth())
			{
				// Heal the player by iRegenAmount health points (will not exceed max health)
				this->TakeHealth((float)iRegenAmount, DMG_GENERIC);
			}
		}
		
		// If we are regenning by at least 1 negative health point, then hurt
		// BUG: if m_flRegenAmount is exactly -1.0f, no hurt occurs (should be: <= -1.0f)
		if (this->m_flRegenAmount < -1.0f)
		{
			// Get just the integer part of the regen amount
			iRegenAmount = (int)ceil(this->m_flRegenAmount);
			
			// Hurt the player by -iRegenAmount health points
			// [ Attacker: self    Inflictor: self    Weapon: none    Force: none    Location: middle of player ]
			this->TakeDamage(CTakeDamageInfo(this, this, nullptr, vec3_origin, this->WorldSpaceCenter(), (float)(-iRegenAmount), DMG_GENERIC));
		}
		
		// SKIPPED: irrelevant stuff
		
		// If we did heal or hurt ourselves, then remove the integer amount we
		// healed/hurt from the future regen amount (but leave any fractional part)
		this->m_flRegenAmount -= (float)iRegenAmount;
		
		// SKIPPED: irrelevant stuff
	}
}